Formation of Brown Dwarfs: Star Formation and the IMF
(Invited review)
Fred C. Adams
The formation of stars, brown dwarfs, and planets is one of the most
fundamental problems in astrophysics. In recent years, a lot of
progress in this area has been made and we now have a successful
paradigm that provides the cornerstone of our current understanding of
the star formation process. Within this paradigm, the agreement
between observations and theory is quite good, especially for the case
of low-mass stars. After reviewing the current theory, we present a
class of models for the initial mass function (IMF) for stars forming
within molecular clouds. This class of models uses the idea that
stars determine their own masses through the action of powerful
stellar outflows. This concept allows us to calculate a
semi-empirical mass formula (SEMF), which provides the transformation
between initial conditions in molecular clouds and the final masses of
forming stars. For a particular SEMF, a given distribution of initial
conditions predicts a corresponding IMF. We can consider several
different descriptions for the distribution of initial conditions in
star forming molecular clouds. In the limit in which many different
independent physical variables play a role in determining stellar
masses, the central limit theorem shows that the IMF approaches a
log-normal form. These results show that this picture of star
formation and the IMF naturally produces stellar mass distributions
that are roughly consistent with observations. This work thus provides
a calculational framework to construct theoretical models of the IMF.
Finally, we review the implications of this theory for the formation
of brown dwarfs, planets, and substellar objects.
Physics Dept., University of Michigan, Ann Arbor, MI 48109, USA
Cool Dwarf Atmospheres: From the Stellar to Substellar Regime
(Invited review)
France Allard
The search for faint (sub)stellar objects as bloomed over the past few
years with the discovery of very low mass stars and cool white dwarfs
in globular clusters, the identification of two young brown dwarfs in
the Pleiades, the discovery of the first evolved brown dwarf Gl229B in
the solar neighborhood, and with several extrasolar jovian-type
planets around nearby solar type stars. Brown dwarfs and extrasolar
giant planets therefore exist and may contribute significantly to the
missing mass in the disk of our galaxy.
While ongoing ground-based and infrared space surveys of unprecedent
faintness and precision continue to reveal dozens more of such
candidates, the presence of first molecules and then condensed
particulates greatly complicates the understanding of their physical
properties. Accurate model atmospheres are necessary for the
calculation of synthetic spectra and the quantitative derivation of
the surface chemical composition. They form a necessary step in the
transformation from theoretical effective temperatures to observed
colors. Proper understanding of their colors is essential in the
search for brown dwarfs. Photospheric models also serve as boundary
conditions for calculations of both stellar interiors and
chromospheres. Generation of frequency-averaged opacities is a
natural by-product of these calculations, and such opacities are
valuable for several fields of research, including star and planet
formation, the formation, structure and evolution of accretion disks,
stellar evolution, and stellar pulsation and dynamics. The properties
of small dust grains, which are present in the atmospheres of cool
dwarfs as well as the interplanetary and interstellar media, play a
critical role in many of these processes.
In this paper, we review the current state-of-the-art of the theory
and modeling of the atmospheres of very low mass stars including the
coolest known M dwarfs, M subdwarfs, and brown dwarfs, i.e. Teff
<= 5,000 K and -2.0 <= [M/H] <= +0.0. We discuss
ongoing efforts to incorporate molecular and grain opacities in cool
stellar spectra, as well as the latest progress in (i) deriving the
effective temperature scale of Mdwarfs, (ii) reproducing the lower
main sequences of metal-poor subdwarfs in the halo and globular
clusters, (iii) the latest results of the models related to the
search for brown dwarfs, and (iv) we present a new grid of model
atmosphere and synthetic spectra for brown dwarfs and giant gas
planets which extends to the conditions of Jupiter's atmosphere.
This research is supported by a NASA LTSA110-96 to WSU and a NASA
EPSCoR grant to the state of KANSAS.
Wichita State University, Dept. of Physics, Wichita, KS 67260-0032
Large Ground Based Telescopes and Extrasolar Planet Detection
(Invited review)
Roger Angel, Buddy Martin, David Sandler and Neville Woolf
Direct detection and crude spectrophotometry of the brighter planets of
nearby stars will be possible with the 6.5 m MMT and Magellan telescopes,
by imaging in the near infrared (Angel 1994, Stahl and Sandler 1995,
Sandler 1997). The stellar halo that normally obscures planets is caused
principally by phase and amplitude fluctuations in the atmospherically
distorted wavefront. The primary step needed for halo reduction is to use
a fast adaptive mirror with many actuators (600/m^2), to make phase
correction to the limiting accuracy imposed by photon noise in the
wavefront sensor. Prediction of the evolving wavefront is needed so that
the residual speckle structure is decorrelated from one correction cycle to
the next, and will average away. The surface of the 6.5 m primary mirror
has been measured, and the fixed pattern halo structure that is on too fine
scale to be corrected is found to be acceptably small. Wavefront amplitude
fluctuations from scintillation will be reduced, and tapered pupil masks
will be used to reduce the light diffracted by the telescope aperture
boundaries. With these steps, Jupiter like planets in 5 AU orbits will be
detectable to 10 pc distance. The technology, performance and costs of the
high resolution deformable mirror will be explored early on, to be sure the
technique will reach the required sensitivity.
Earth-like planets should be directly detectable in the thermal infrared by
the Planet Finder space mission, but only if zodiacal clouds of other stars
are not much brighter than in the solar system. The ground based
interferometer with the highest sensitivity to survey nearby stars for such
faint clouds is the Large Binocular Telescope (LBT) being built by the USA,
Italy and Germany (Woolf and Angel 1995, Angel and Woolf 1997). Its
sensitivity derives from its uniquly favorable configuration for
interferometric nulling, with two 8.4 m primaries closely and rigidly
mounted to a beam held perpendicular to the line of sight, as for the
nulling configuration of Planet Finder. Thermal background, which limits
sensitivity, is minimized in the LBT by the individual beam profiles which
are matched to the expected size of the dust cloud (about 0.25 arcsec), and
by the minimal number of emissive surfaces in the optical train. Through
its use of adaptive secondaries and a cryogenic beam combiner, and because
beam combination is direct for the co-mounted mirrors, only three warm
surfaces are needed, and the LBT's sensitivity will just reach clouds at
the solar system level. The cryogenic beam combiner, which must null to a
part in 10^4, will be developed in a test bed configuration before
installation in the telescope.
Center for Astronomical Adaptive Optics, Steward Observatory, University of
Arizona, USA
On the Formation of Eccentric Superplanets
Pawel Artymowicz
Several classes of the extrasolar planets are emerging, as
reviewed by Marcy & Butler at this conference: four 51
Peg-type planets with ~ Jovian mass and small circular
orbits, one planet in a wide circular orbit resembling
Jupiter (47 UMa), and three puzzling planets on very
eccentric orbits. Two of the eccentric planets
appear to have been born
this way (70 Vir and HD114762) and one may have a stellar
companion-induced eccentricity (16 Cyg B). 70 Vir and
HD114762 have significantly super-Jovian minimum masses of
order 10 Jupiter masses, and can thus be called
"superplanets". I shall present some attempts at
understanding the origin of such bodies.
Are they really planets (i.e., have they formed in solar
nebulae, like planets) or brown dwarfs? And if they are
planets, why do they differ so much from the rest? The
pre-1996 theory of planet formation did not, as a rule,
consider the formation of superplanets, let alone the ones
on elongated orbits. One notable exception from this rule
will be discussed: a theory of resonant disk-planet
interaction predicting that any sufficiently massive stellar
companion (whether a star or a superplanet of >10 Jupiter
masses) interacting with protostellar accretion disks will
rapidly be transferred onto an eccentric orbit, while less
massive bodies (planets) will be circularized by the gravity
(not the gas drag) from the disks. New work in this area
suggests dynamical pathways leading from planets to
superplanets, but does not yet answer the question of
whether brown dwarfs may form in a similar way.
Recent alternative theories such as the planet-planet
interaction theory will also be discussed and evaluated in
the light of the empirical mass-eccentricity relationship
for stars and planets (cf. Mayor, Queloz, & Udry, and
Latham, this conference).
Stockholm Observatory, Stockholm University, Sweden
Evolution and Structure of Brown Dwarfs
(Invited review)
I. Baraffe
Over the past decade considerable effort, both observational and
theoretical, has been directed towards deriving a more accurate
determination of the stellar lower main sequence and of
the sub-stellar domain covered by Brown Dwarfs. The main theoretical
efforts have been devoted to the equation of
state (EOS), screening factors of thermonuclear reactions and
non-grey atmosphere models, and now yield
a better understanding of such objects.
Modeling of the inner structure of Brown Dwarfs requires first a correct
description of non-ideal effects in dense plasmas, as well for the equation
of state
(EOS) as for the screening factors of the nuclear
rates. Only recently has a new EOS been derived, devoted to the
description of such compact objects, which presents a consistent treatment
of pressure ionization.
Moreover, due to strong non-grey effects, an accurate surface
boundary condition based on non-grey atmosphere models is required for
the derivation of reliable evolutionary models.
Improvement in the modeling of cool non-grey atmospheres, extremely complex
due to the presence of a wide variety of molecular absorbers and condensates
has blossomed in recent years.
The present talk will focus on the most recent
progress in the field and their implications on
evolutionary models of Brown Dwarfs.
Ecole Normale Supèrieure de Lyon - CRAL - 69364 Lyon - France
Spectroscopic Confirmation and Analysis of Brown Dwarfs
(Invited review)
Gibor Basri
There are 3 ways in which a brown dwarf may be certified as such:
dynamically, by its temperature and luminosity, and spectroscopically. Each
works best in a certain domain; brown dwarfs have now been identified by all
3 methods. I discuss the spectroscopic identification of young brown dwarfs,
principally by the ``lithium test''. Following a brief summary of the
theory, I review the work that has been done so far. After an inital period
of frustration, there have lately been a number of substellar objects
certified by this method, both in clusters and in the field. The test,
combined with the object's luminosity and temperature, also imparts
constraints on its age. In order that the results in the Pleiades make
sense, the cluster must be substantially older than implied by the
``classical'' upper main sequence turnoff. Future prospects for work in this
area are exciting. I also briefly discuss another spectroscopic test, the
presence of methane which indicates substellar atmospheric temperatures (as
in Gl 229B). Finally, I summarize what has been discovered spectroscopically
about the angular momentum and magnetic activity evolution near the
substellar boundary.
Astronomy Dept., Univ. of California, Berkeley, CA 94720, USA
Is PPL 15 a Binary Brown Dwarf System?
Gibor Basri (1), Eduardo L. Martín (2)
In a recent preprint, Zapatero-Osorio, Martín and Rebolo present
infrared photometric evidence suggesting that the first ``lithium brown dwarf'',
PPL 15 (Basri, Marcy, Graham 1996), might actually be a binary. That is
because it is too bright and red to lie in the single main sequence in
the cluster, but sits right on the boundary of the binary main sequence
proposed by Steele and Jameson (1995). The suggestion is made that
PPL 15 could be a system with 2 brown dwarfs.
A planetary camera image of PPL 15 in
the V band with HST, eliminates the presence of a companion with a
contrast up to 5 magnitudes and a separation greater than 0.15 arcsec
(19 AU).
The HIRES spectra used to detect the presence of lithium were
obtained over 3 consecutive nights in November 1994 and 2 consecutive
nights in March 1995. They are therefore useful in a radial velocity
variability study at both very short and intermediate timescales. They
are all noisy, and of varying quality, but certainly useable in orders
well redward of lithium. We have re-examined this data, and find that
PPL 15 is a double-lined spectroscopic binary. On all but the first
night of observation, a cross-correlation between PPL 15 and an dM6.5 spectral
standard produces a double peak, with a separation of roughly 30-35
km/s. The first night is single-lined, and cross-correlations between
it and the following nights reproduce the same double peaks. We show
that there are plausible orbital solutions which can reproduce our
results, suggesting a very short period (of order one week). It will be
very important to obtain further data to determine an orbit solution
for the system.
The mass of PPl 15 had been estimated at ~78 Jupiter masses assuming
it was a single star, but its double-lined spectroscopic binary nature
(if confirmed) would imply masses less than 65 M_J for each component.
References:
(1) Department of Astronomy, University of California, Berkeley, CA 94720, USA
(2) Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
G. Basri, G.W. Marcy, J.R. Graham 1996, ApJ, 458, 600
I.A. Steele, R.F. Jameson 1995, MNRAS, 272, 630
M.R. Zapatero Osorio, E.L. Martin, R. Rebolo 1997, AA, in press
Effects of Accretion on the Binary Mass Ratio Distribution: Implications for
Brown Dwarfs in Binaries
Matthew R. Bate
We consider the effects of accretion
during binary star formation on the
mass-ratio distribution of binaries.
When a `seed' binary system forms within
a collapsing molecular cloud core, it
initially contains only a small fraction
of the total cloud mass. The system must
grow in mass via accretion of the remaining
cloud material.
Recent numerical calculations have
determined the how the mass ratio of a
protobinary system is modified by accretion.
Using these results, and given the initial
density and angular momentum profiles of the
molecular cloud core in which a binary forms,
we can determine the evolution of a binary's
mass ratio as it accretes from its initial to
its final mass.
We find that the more material that is accreted,
relative to the binary's initial mass, the
closer the mass ratio becomes to unity. Thus,
close binaries are more likely to have mass
ratios near unity than wide binaries. In order
for a low-mass-ratio, short-period system to be
formed (e.g. a brown dwarf and a solar-type star
in a system with less than 10 AU separation), the
secondary must be formed near the end of the
accretion phase to avoid its mass being increased
relative the the primary.
Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg, Germany
Interfero-Coronagraphy: a tool for detection of faint companions
Pierre Baudoz (1), Yves Rabbia (2) and Jean Gay (1)
We present a concept of interfero-coronagraph which should allow to
images faint companions using a single telescope. Applications to diluted
pupils is also conceivable.
This coronagraph is based on the principle of destructive interferences to get
light from a star on-axis down to zero in the whole focal plane without
destructing light from the surrounding environment of the main star.
Angular resolution of the recorded images is then limited only by the size of
the aperture, since no material mask is used (like in the classical Lyot's
coronagraph apply to stellar imaging). Detection of faint companions as closer
to the main star as the first diffraction ring is then achievable.
The main constraint for operation of our coronagraph is that the complex
amplitude must be centro-symmetric.
Departure from this situation makes the extinction of the main star uncompleted
and thus lowers detection capabilities.
Observing from the ground requires adaptive optics, but using space telescopes
enhances widely the capabilities of the instruments because space satisfy the
constraint almost perfectly (limitations come then from the imperfections of the
optical surfaces).
Results of numerical simulations are given, showing the effect of incomplete
correction by adaptive optics ( ground-based operation).
We present expected limits of our coronagraph regarding detection of brown
dwarfs or exoplanets from ground-based telescope.
We eventually describe our prototype, currently under development at the
Observatoire de la Côte d'Azur.
(1) Observatoire de la Côte d'Azur (O.C.A.), Département Fresnel,
URA CNRS 1361, BP 04229, 06304 Nice Cedex 4, France
(2) .C.A., Dpt. Fresnel, URA CNRS 1361, avenue Copernic, 06130 Grasse, France
Stable planetary orbits in double star systems
Daniel Benest
Cosmogonical theories as well as recent observations allow us to expect the
existence of planets around many stars other than the Sun. On an other hand,
double and multiple star systems are established to be more abundant than
single stars (such as the Sun), at least in the solar neighborhood. We are then
faced to the following dynamical problem: assuming that planets can form in a
binary early environment (I do not deal here with), does long-term stability
for planetary orbits exist in double star systems?
Although preliminary studies were rather pessimistic about the possibility of
existence of stable planetary orbits in double or multiple star systems, modern
computations have shown that many such stable orbits do exist (but possible
chaotic behaviour), either around the binary as a whole (P-type) or around one
component of the binary (S-type), this latter being explored here.
The dynamical model is the elliptic plane restricted three-body problem; the
phase-space of initial conditions is systematically explored, and limits for
stability have been established. Stable S-type planetary orbits are found up to
distance of their ``sun" of the order of half the periastron distance of the
binary; moreover, among these stable orbits, nearly-circular ones exist up to
distance of their ``sun" of the order of one quarter the periastron distance of
the binary; finally, among the nearly-circular stable orbits, several stay
inside the ``habitable zone", at least for two nearby binaries which components
are nearly of solar type.
CNRS URA 1362, O.C.A. Observatoire de Nice, B.P. 4229, F-06304 Nice Cedex 4 (France)
Gravitational Microlensing Results on Brown Dwarfs and Planets
in the Milky Way Halo, Disk, and Bulge
(Invited review)
D. Bennett
Gravitational microlensing is a technique which can detect distant
compact objects by virtue of their gravitational field, so it is sensitve
to isolated objects such as planets and brown dwarfs which do not emit
a detectable amount of light. Microlensing searches toward the Large
Magellanic CLoud by the EROS and MACHO collaborations have ruled out
planetary mass objects as major contributors to the mass of the dark halo,
but they have detected a new population of objects that appear to contribute
a substantial fraction of the total dark halo mass. Microlensing searches
toward the Galactic bulge by the MACHO and OGLE collaborations have also
turned up an unexpectly large number of microlensing events caused by lensing
objects in the Galactic disk and bulge. A few of these events seem to
indicate lenses with planetary masses.
With the advent of microlensing follow-up programs, microlensing is also
becoming a powerful tool that can be used to do a sensitive search for
planets around other stars. Future microlensing follow-up efforts may be
able to mount a sensitive planetary search capable of detecting planets
down to an earth mass.
University of Notre Dame. Physics Department, 46556 Notre Dame, IN, USA
Lithium Depletion in Fully Convective Pre-Main Sequence Stars
Lars Bildsten, Edward F. Brown, Christopher D. Matzner, and
Greg Ushomirsky
We present an analytic calculation of the thermonuclear depletion of
lithium in contracting, fully convective, pre-main sequence stars of
mass M < 0.5 Msun. Previous numerical work relies on
still-uncertain physics (atmospheric opacities and convection, in
particular) to calculate the effective temperature as a unique
function of stellar mass. We assume that the star's effective
temperature, Teff, is fixed during Hayashi contraction and allow
its actual value to be a free parameter constrained by observation.
Using this approximation, we compute lithium burning analytically and
explore the dependence of lithium depletion on Teff, M, and
composition. Our calculations yield the radius, age, and luminosity of
a pre-main sequence star as a function of lithium depletion. This
allows for more direct comparisons to observations of lithium depleted
stars. Our results agree with those numerical calculations that
explicitly determine stellar structure during Hayashi contraction. In
agreement with Basri, Marcy, and Graham (1996), we show that the
absence of lithium in the Pleiades star HHJ 3 implies that it is older
than 100 Myr. We also suggest a generalized method for dating galactic
clusters younger than 100 Myr (i.e., those with contracting stars of
M > 0.08 Msun) and for constraining the masses of lithium
depleted stars.
Department of Physics and Department of Astronomy,
University of California, Berkeley, Berkeley, CA 94720, USA
The dark-speckle method for imaging extrasolar planets
Anthony Boccaletti
Searching for faint companions like brown dwarfs and exoplanets has become
a priority in scientific research. It has been shown that a classical long
exposure, even with HST, is not enough efficient to detect these kind of objects.
The "dark-speckle method" (Labeyrie, 1995) is applicable with large
ground-based telescopes to remove the scattered light near a stellar image,
beyond the limits of current adaptive coronagraphic systems. It exploits the
darkening occuring randomly in the stellar speckles forming the residual halo,
not removed by the adaptive coronagraphic stage of the dark-speckle camera.
A planet can become detected when the stellar halo overshining it is briefly
and locally cancelled by the formation of a dark speckle. The algorithm
utilizes a series of short exposure, recorded with a photon-counting camera,
to generate a cleaned image where the visibility of circumstellar bodies is
markedly enhanced. Calculations of the signal/noise ratio indicate that planets
at 10^-9 relative luminosity may become detectable in a few hours with large
aperture (Boccaletti et al, 1997). Preliminary results of simulations and
observations, with photon-counting devices (camera and avalanche photodiode)
are consistant with the model. Under laboratory simulation, a 10^-6 companion
is detected at 5 Airy radii from the star. It is unclear yet whether CCDs and
infra-red detectors are also usable. A variant of the method is also considered
for an HST instrument.
"Images of exo-planets obtainable from dark speckles in adaptive telescopes",
Labeyrie, A., 1995, Astron. Astrophys., 298, 544.
Observatoire de Haute Provence and Observatoire de Meudon (DESPA)
Boccaletti, Ragazzoni & Labeyrie, - in preparation
Formation and Evolution of Substellar Objects Orbiting Stars
(Invited review)
P. Bodenheimer
Substellar objects orbiting stars can be formed by (1) a process
of direct fragmentation during the collapse of a rotating interstellar
cloud in which a system of small mass ratio (say 1:10) is formed,
(2) by gravitational instability in an equilibrium disk which
has condensed out of the collapse, (3) by fragmentation of a
collapsing cloud into a multiple system, followed by capture of a
low-mass fragment by a high-mass fragment, or (4) by accretion of solid
particles in a disk to form a solid core of a few earth masses,
followed by capture of gas. In cases (1) and (3) the object would be
called a brown dwarf, in case (4) it would be called a planet, and in
case (2) its identity would be controversial. The arguments for and
against these various scenarios will be discussed. Possible formation
processes for the newly discovered extra-solar 'planets' will be
discussed. New calculations will be presented for the collapse and
fragmentation of rotating interstellar clouds, which could produce
brown dwarfs according to case (3); and for the formation of giant
planets according to case (4).
Case 1. Direct fragmentation............. unequal mass fragments
once formed tend to equalize as a consequence of accretion.
Case 2. Disk fragmentation.............. disk may transfer mass and
angular momentum through low-amplitude spiral waves and never get to
the stage where Q gets low enough to result in fragmentation.
Case 3. Multiple fragmentation.....Low mass fragments are produced in
numerical simulations but their long-term evolution has
not been calculated. Mergers and further accretion will
take place, so final masses not determined.
Case 4. Standard giant planet formation picture.
UCO/Lick Observatory, Board of Studies in Astronomy
and Astrophysics, University of California, Santa Cruz, CA 95064, USA
Bonnell and Bastien show that only a small initial
perturbation will result in a fairly extreme mass ratio.
Opacity limited fragmentation applies
Look up objections to the `GGP' model.
Formation time is fast.
Need estimates of Jeans masses in various types of disks.
Would the masses tend to be larger than typical giant planet masses.
Adams and Benz calculation shows that fragmentation is possible, apparently
with an eccentric orbit, but an m = 1 perturbation had to be included, and
boundary effects may be important.
Alan Boss's calculation ... resulted in about 10 M_J planets.
Opacity limited fragmentation applies.
Tidal truncation limit.
Formation times.
Migration.
An HST/NICMOS and ESO/Adaptive Optics search for young brown dwarfs and giant planets
Wolfgang Brandner (1), Hans Zinnecker (2) and France Allard (3)
Contrary to classical T Tauri stars weak-line T Tauri stars no longer possess
massive circumstellar disks. In weak-line T Tauri stars, the circumstellar
matter was either accreted onto the central star or redistributed to form
planetesimals, which eventually may form giant planets and brown dwarfs.
YOUNG low-mass companions (ages 2-20 Myr) are considerably brighter than
old low-mass companions (i.e. brown dwarfs or giant planets around MS stars),
and hence are much easier to detect. Model calculations (cf. invited
review by F. Allard, this conference) show that the spectral energy
distribution of young brown dwarfs and giant planets peaks at 1.1 microns
almost independently of their effective temperature. Therefore observations
at 1.1 microns are best suited for detecting this kind of objects.
Based on photometric and spectroscopic studies of ROSAT sources
we have selected an initial sample of 200
weak-line T Tauri stars in the Chamaeleon T association and the
Scorpius Centaurus OB association. In the course of follow-up observations we
identified visual and spectroscopic binary stars and excluded them from our
final list as the complex dynamics and gravitational interaction in binary
systems might aggravate or even completely inhibit the formation of planets
(depending on physical separation of the binary components and their
mass-ratio). We ended up with a final sample of about 70 presumably single
weak-line T Tauri stars.
Now we initiated a programme to spatially RESOLVE young brown dwarfs and
young giant planets as companions to single weak-line T Tauri stars
using HST/NICMOS and ground based adaptive optics at the ESO 3.6m
telescope.
The ground based adaptive optics observations of about 30 G and K type
weak-line T Tauri stars will begin by the end of March 1997. From July 1997
on 24 M type weak-line T Tauri stars will be surveyed with HST/NICMOS
for faint brown dwarf or giant planet companions. Depending on
the brightness difference between the primary and the low-mass companion our
survey will be sensitive to separations down to 0.20'', i.e. 30 AU
(comparable to the semi-major axis of the orbit of Neptune) at a distance of
150 pc. Thus it will probe planetary systems in a range of separations
where radial velocity methods are not sensitive for.
Our programme might provide first observational estimates on the
frequency of brown dwarfs or giant planets around young late-type stars and
hence on the overall frequency of planetary systems.
(1) University of Illinois at Urbana-Champaign, Department of Astronomy,
1002 West Green Street, Urbana, IL 61801, USA
(2) Astrophysikalisches Institut Potsdam, An der Sternwarte 16,
14482 Potsdam, Germany
(3) Wichita State University, Department of Physics, 1845 Fairmount,
Wichita, KS 67260-0032, USA
Deep CCD Imaging in the Taurus clouds
César Briceño (1,2,3) and Lee W. Hartmann (1)
We present preliminary results of our deep CCD VRI survey of the six
groupings identified by Gomez et al. (1993, AJ, 105, 1927) in Taurus.
These groups contain about > 60% of all T Tauri stars in Taurus,
so by probing these selected fields for very low-mass young stars we
should be able to obtain a sample which is representative of the
low-mass end of the IMF in the Taurus star-forming region.
Our approach uses large-format CCD images to identify faint, very red
potential low-mass candidate T Tauri stars over a relatively wide area.
This approach is reasonably useful because Taurus has
a relatively low extinction for a star-forming region (A_V ~ 1 - 2
for many T Tauri stars). Three color photometry allows us to construct
color-magnitude and color-color diagrams for each survey field and use
these to select candidate stars from the reddest and fainter objects.
We then conduct spectroscopic follow up observations of CCD survey candidates
so the true nature of these objects can be established.
(1) Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge,
Massachussets 02138, USA
(2) Also at Postgrado en Física, Universidad Central de Venezuela, Aptdo.
Postal 20513, Caracas 1020A, Venezuela
(3) Also at Centro de Invesitigaciones de Astronomía (CIDA), Aptdo. Postal
264, Mérida 5101-A, Venezuela
Theory of the Structure and Evolution of Extrasolar Planets and Brown Dwarfs
(Invited review)
Adam Burrows
With the discovery of the companions of 51 Peg, 55 Cnc, tau Boo,
upsilon And, 70 Vir, 47 UMa, and Gl229, evolutionary and spectral models
of gas giants and/or brown dwarfs with masses from 0.3 through 60 times that
of Jupiter assume a new and central role in the emerging field of extrasolar
planetary studies. In this contribution, I describe the structural,
atmospheric, and evolutionary characteristics of such exotic objects, as
determined by our recent theoretical calculations. The issue, in part
diversionary, of what distinguishes a brown dwarf from a giant planet will
be addressed, as will the luminosities, effective temperatures, gravities,
colors, and spectral signatures of these beasts as a function of age and mass.
Our theoretical calculations can be used to establish direct search strategies
via SIRTF, ISO, and HST (NICMOS) and via various ground-based adaptive optics
and interferometric platforms planned for the near future.
University of Arizona, Tucson, AZ, USA 85721
The Keck Doppler Search for Planets
R.P. Butler, G.W. Marcy
In 1996 July we began a Doppler survey of 400 G
and K main sequence dwarf stars with Keck 1
and the HIRES spectrometer. The high S/N
of obsevations made with the Keck (~400)
will allow photon limited precision of 2 m/s,
sufficient to render a 6 sigma detection of a
Jupiter mass planet and a 2 sigma detection of
a Saturn mass planet at 5 AU.
Preliminary work has concentrated on sample
selection and developement of a HIRES model,
including the variable point-spread-function,
capable of removing all instrumental effects
at the level of one part in one-hundred million.
The primary goal is to carry out a statistically
meaningful survey of solar type stars, with
sensitivity sufficient to detect true solar
system analogs. This is required to construct
a Planet Mass Distribution Function and to
answer basic questions such as, "What fraction
of solar type stars have planets?", and "What
fraction of planetary systems are similar to
the Solar System?".
San Francisco State University, University of California,
Berkeley, 94720 CA, USA
An estimate of the total mass of the Galactic Halo in substellar objects, from an observationally tested chemical evolution model
E. Casuso, J.E. Beckman
The recently established observational plots of Be and B
versus Fe in the solar neighbourhood, extending from metallicities
close to solar down to metallicities below [Fe/H] = -3 have led us
to formulate in detail a new chemical evolution model for the
solar neighbourhood (Casuso and Beckman, 1997). This model, which
includes both the halo and the disc, gives an excellent account of
the plot of local stellar numbers as a function of metallicity
(exemplified in the "G dwarf problem") and as well as explaining
the plots of Be and B vs Fe, describes very well the behaviour, using
Fe as a benchmark of O, N, and C, and the C12/C13 ratio.
One of the features of that part of the model which describes
the chemical evolution of the halo is the clear requirement that
a major fraction of the baryonic halo mass be stored in objects of
relatively low mass. Detailed modelling shows that for the halo the
observations are not as well fitted by an IMF which flattens at
low masses as by a classical Salpeter power law, and that some 60% of
the condensing mass must form sub-stellar objects, with a lower mass
limit close to 0.01 Msun. These are sample numbers from an understandably
simplified model, but the basic necessity for a major mass fraction,
of order one half of the total mass, to be locked up in sub-stellar
objects is an inevitable result in any model capable of explaining
the sudden rise in the observed metallicity distribution of halo
stars close to [Fe/H] = -1.6, as well as the high observed Be
and B abundances in stars in the range -3 < [Fe/H] <-2. This implies
an upper limit of 10^8 Msun to the total mass of objects in the mass
range 0.01 Msun < M < 0.1 Msun, which means that these objects form the
dominant component of the baryonic halo mass (compare estimates of
the total gasesous mass of 10^6 Msun). It must be noted, however, that
10^8 Msun is only 0.1% of the mass which would be required to yield
the flat rotation curve observed for the Galaxy at large
galactocentric radii.
Instituto de Astrofísica de Canarias. E-38200 La Laguna, Tenerife. Spain
Role of Brown Dwarfs in the determination of the luminosity function of young stellar populations
C. Censori, and F. D'Antona
We present theoretical luminosity functions (LF) in the IR bands J and
K, based on D'Antona and Mazzitelli (1994, DM) models, but allowing for
variational assumptions on the deuterium abundance, on the slope of the
initial mass function and on the rate of star formation. The peaks due to
deuterium burning are very important for coeval systems. We make a few
comparisons with the LFs of the L1641 regions observed by Strom, Strom and
Merrill (1993). The main results are:
Osservatorio Astronomico di Roma, I-00040 MONTE PORZIO, Italy
1.- Consideration of masses M < 0.1 Msun is necessary to describe
correctly the low luminosity LF at all ages, and mainly for the older
populations.
2.- Luminosity functions derived from tracks in which D-burning is
suppressed do not reproduce the observations.
3.- A deuterium abundance X_D ~ 4x10^{-5} might represent a better
initial value to reproduce the observed LFs, although DM models
were computed with a deuterium abundance in mass fraction X_D ~ 2x10^{-5},
which is well representative of the D-abundance in the local ISM.
ISO observations of brown dwarf candidates in Rho Ophiuchi
F. Comeron (1), P. Claes (2)
We present the results of observations of
candidate brown dwarfs in the Rho Ophiuchi
embedded cluster in the 3-8 micron domain
carried out with ISOCAM. The candidates were
selected on the basis of their near-IR
photometry obtained in ground based surveys.
The extension of the photometry to the
domain covered by our ISO observations
allows a much better sampling of the spectral
energy distribution in the
regions where photospheric and circumstellar
emission are expected to dominate. The
intrinsic spectral energy distributions of the
embedded objects can be reliably estimated in
this way, and comparison to theoretical models
of pre-main sequence evolution can then be used
to evaluate masses. In this way, the substellar
nature of our targets, suspected from the
ground based observations, is strongly supported
by the new results from ISO.
Discovery of the lowest mass Brown Dwarf in the Pleiades
M.R. Cossburn, S.T. Hodgkin, R.F. Jameson, D.J. Pinfield
We have imaged the Pleiades open cluster
at I and Z in a search for
low-mass stars and brown dwarfs. One very
red object, which we have called PIZ 1, at
I=19.64, I-Z=1.33 has been detected within
an area of 100 square arcminutes.
Follow up infrared photometry verifies that
this object is extremely red with a K
magnitude of 15.5 . We have also obtained
a spectrum which exhibits the spectral features
indicative of an extremely cool M dwarf.
We estimate the effective temperature and
mass of PIZ 1 to be 2300 K and 0.048 Msun
respectively.
High-resolution Ground-based Spectroscopy of 51 Peg B:
Search for Atmospheric Signatures
A. Coustenis, J. Schneider, R. Wittemberg (1), E. Chassefière (2),
T. Greene (3), A. Penny (4), and T. Guillot (5)
The planet in orbit around 51 Peg has, according to theoretical
models, moved into its current position from farther away and
its atmosphere may therefore be strongly evaporating. The
molecules released by the upper parts of the atmosphere must
be rapidly ionized and could form an envelope around the
planet. If the planet passes in front of the star, this
envelope is in favorable geometric conditions (the inclination
angle of the orbit is close to 90 degrees) to cause a
star occultation. Absorption lines due to the signatures
of atmospheric ions could then be observed.
We have observed 51 Peg alone and during the transit of the planet
in front of the star on Aug. 18, 19, 27, 29, Sept. 3 and 4 1996
with CSHELL at IRTF and CGS4 at UKIRT with resolving powers
of the order of R=20000. Our data have a S/N ratio
of about 500 and are currently under calibration. In the course of the
analysis we will search for absorption lines from the material evaporated
around the planet while occulting the star that would enable us to infer
information on the nature of the atmosphere around the planet.
We will present preliminary results.
A role for superadiabatic convection in low mass stars structure?
F. D'Antona and I. Mazzitelli
We compute the structure of stars of mass M <= 0.5 Msun starting from
the pre-main sequence and employing different assumptions on the comvection
model, in particular dropping the Mixing Length Theory (MLT) and adopting a
Full Spectrum of Turbulence (FST) modelization (Canuto and Mazzitelli 1990,
Canuto, Goldman and Mazzitelli 1996) convection in the upper stellar envelope
is so inefficient that there is a small superadiabatic layer. According to
the detailed treatment of the convection model, then, the resulting
Teff may differ up to ~ 100 K, a result which can not be found by
changing the l/H_P value in the MLT models within accepted values. It
would then be important to introduce the FST treatment of convection in
model atmospheres which are necessary to a full understanding of the
observational characteristics of of low mass stars.
The occultation method as a way to detect extrasolar planets
Hans-Joerg Deeg
The occultation method for the detection of extrasolar planets is
based on the detection of stellar brightness variations which result
from the transit of a planet across a star's disk. This method is
especially suited for the detection of planets in the habitable zone,
as occultations can be detected for planets close to the central
star. Also, in suitable systems, planets in the Earth-to-Neptune size
range are detectable with terrestrial 1m class telescope. Strategies
to improve detection probabilities are the observation of stars with
inclinations close to 90 degrees, especially binary eclipsing stars,
and the use of matching filter algorithms to extract sub-noise transit
signals. An overview over current earth-based, as well as future space
based observing programs will be given. The occultation method may
also serve to observe atmospheric features in extrasolar planets,
which may be indicative of exobiologic processes.
The rotation of the M dwarf in the solar neighbourhood
Xavier Delfosse (1), Thierry Forveille (1), Christian
Perrier (1), and Michel Mayor (2)
We have obtained high resolution spectra for 115 out of 121 stars in a
volume limited sample of field M dwarfs. We use these observations to
derive rotational velocities, and fluxes in the Halpha and
Hbeta lines. 9 of the stars are double-lined spectroscopic
binaries with measured or probable periods short enough that rotation
may be tidally synchronized with the orbit, and another 7 are visual
binaries in which we cannot disentangle the lines of the two stars. Of
the remaining 99 stars, 24 have rotational velocities above our
detection limit of 3 km/s, and three are fast rotators, two with
vsini = 30 km/s and one with vsini = 50km/s. Given the
small radii of M dwarfs these moderate rotation velocities correspond
to rather short rotation periods of 7-8 hours.
Rotation is strongly correlated with both spectral type and kinematic
population: all stars with measurable rotation are later than M3.5,
and all but one have kinematic properties typical of the young disk,
or intermediate between the young disk and the the old disk. We
interpret this correlation as evidence for a spin-down timescale that
increases with decreasing mass. At the age of the thick disk and halo,
all stars earlier than M5-M6 (0.1-0.15 Msun) have spun-down to
below our detection limit. At the age of the young disk this has only
happened for stars earlier than M3.5. The one star with measurable
rotation and a kinematics intermediate between old disk and halo has
spectral type M6, still consistent with this general idea. It could
also belong to the tail of the velocity dispersion distribution of a
slightly younger population. On the other hand, we observe no
conspicuous change in the vsini distribution at the mass
(M~0.3 Msun) below which stars remain fully convective down
to the main sequence.
There is a saturated correlation between rotation and activity for
this sample, consistent with the one observed for younger or more
massive stars: Lx/Lbol and LHalpha/Lbol both
correlate with vsini for vsini < 5 km/s and then
saturate at respectively 10^{-2.5} and 10^{-3.5}.
Ground-Based Detection of Terrestrial and Jovian Planets Around Eclipsing Binaries
Laurance R. Doyle
Small eclipsing binary systems provide encouraging sites for the
photometric search for extrasolar planets for four major reasons: 1) Their
small sizes allow planetary transits to be more easily detected, 2) Their
binary mass distribution causes any protoplanetary material to precess at
different rates depending on orbital radii so that material may be expected to
be damped into the binary orbital plane, that is, planets will form in the
line-of-sight plane, 3) If planets form in the same thermal regime as the
solar system then such low luminosity stars could form terrestrial-type
planets with periods of weeks rather than months, and 4) a planetary transit
across the changing phase configuration of an eclipsing binary is uniquely
quasi-periodic allowing a matched-filter cross-correlation with models that
should be able to extract sub-noise planetary transit features unambiguously.
Also in the photometric data are precisely timed eclipse minima that can
reveal the presence (or absence) of giant planets or brown dwarfs by showing a
drift in the eclipse minima epochs that is periodic. We report on an ongoing
project to detect sub-Neptune-sized planets around CM Draconis (the smallest
known eclipsing binary) and constrain the existance of giant planets around
it and about one dozen other small mass systems.
Systematic search of nearby faint stars
Ducourant C. (1), Dauphole B. (1), Rapaport M. (1),
Colin J. (1), and Geffert M. (2)
We present here a long term project that has been undertaken at Bordeaux
Observatory for several years. The aim of this work has been to
make systematic search for nearby faint stars using Schmidt plates
and to accurately measure the parallax of these good candidates using
CCD observations. We will show how we have been able to detect 6 new
faint objects closer from sun than 50 parsecs.
Brown dwarfs in the Pleiades, a deep IJK survey
Leif Festin
An area large enough (180 arcmin^{2}) to put constraints on a possible
low mass brown dwarf population in the Pleiades has been surveyed to
very faint magnitudes in I, J and K. The completeness limit, I=21.6,
corresponds to a mass of 0.01 Msun for a cluster age of 70 Myr and
0.035 Msun for 120 Myr. No low mass brown dwarf was found, which is
consistent with previous investigations at higher masses that the brown
dwarf initial mass function is a m^{-1}, or even less steep power law.
Thus low mass brown dwarfs cannot contribute significantly to the Pleiades'
mass. One new possible Pleiades member was found, mass ~ 0.08 Msun (age
120 Myr).
The agreement of the data with the most recent theoretical colour-magnitude
sequences is excellent. All previously known Pleiades in the field fit
in very nicely. Thanks to the excellent seeing (median ~ 0.6") during the
observing runs, there is not a single contaminating background object
within the 180 arcmin^{2} field. It is shown that the most reliable and
time efficient tool for extracting the cluster sequence photometrically
from background stars is to use a I vs I-J colour magnitude diagram. To
improve the statistics, this program was recently extended to include
another 460 arcmin^{2} in I and J + 200 arcmin^{2} in R and I.
The DENIS Sky-Survey, First Results on Very Low Mass Stars and
Substellar Objects
T. Forveille (1), X. Delfosse (1), C. Tinney (2)
We present the first results of DENIS (a DEep Near Infrared Southern
sky survey) on the disk population of very low mass stars and brown
dwarfs. DENIS is an ongoing three colour survey of the whole southern
sky, with limiting magnitudes of I=18.5, J=16, and K=13.5. This
colour combination is ideal to search for objects at the bottom of the
main sequence and beyond: I-J is very sensitive to effective
temperature in the relevant range, and J-K can be used to exclude
reddening as an explanation for an extreme I-J.
The sensitivity and broad sky coverage of DENIS thus makes it an
excellent tool to study the local field population of these objects:
DENIS can detect an M9 dwarf until 40-50pc, the brown dwarf candidate
GD165B would be visible until ~ 15pc, and the confirmed brown
dwarf Gl 229B until 6.5pc.
DENIS observation have begun in December 95 and they have up to now
(december 96) covered over 20% of the southern sky. The results
presented here are based on preliminary processing of about 250 square
degrees, a small fraction of the existing sky coverage. They reveal a
sizeable population of very cold red dwarfs or massive brown
dwarfs. We have in particular discovered a few field objects which are
at least as red as GD165B, and significantly brighter (thus
presumably closer). Infrared spectra of these objects have been
obtained at the AAT, and we are measuring CCD parallaxes for them.
These results use under 2% of the high galactic latitude sky that
will be covered by DENIS. Extrapolation to 15000 square degrees shows
that the survey will detect a few hundred objects similar to GD165B.
They will be close, and their parallaxes will be easily mesured, so
that their luminosity function will be directly determinable, without
the uncertainties inherent in the use of photometric distances. DENIS
should thus definitely settle the shape of the disk luminosity function
down to at least this spectral type. Significantly fainter objects
will presumably be found too, with their number depending on the slope of
the luminosity function at I-J > 3.5.
M-subdwarfs and the galactic halo mass function
John E. Gizis
We present preliminary results of a proper motion
survey designed to measure the mass function of
field metal poor stars as a function of metallicity.
The mass function and its extrapolation down to
substellar masses constrains star formation theories
that predict large numbers of brown dwarfs
in metal-poor enviroments. Spectra and photometry
of nearby metal-poor stars is presented in order
to establish the properties of very cool
metal-poor stars at the hydrogen burning limit.
Hyades Binaries and (no) Brown Dwarfs
J. Gizis, N. Reid
We present an imaging survey of Very Low Mass Hyads
using the WFPC2 camera of HST. Our aim
is to determine the binary
fraction at the bottom of the main sequence.
We find that only ~ 20% of the targets have
resolvable stellar companions, in agreement with
expectations from the field M-dwarf sample.
Due to the young age of the Hyades cluster,
high mass brown dwarfs should be detectable, yet
we find none. We discuss the resulting constraints
on the companion mass function across the hydrogen
burning limit. We also consider the effects of
binaries on the determination of the stellar
luminosity and mass functions in the Hyades.
The Stellar Metallicity - Planet Connection
Guillermo González
The parent stars of the recently announced extrasolar planetary
candidates are far from typical in terms of their chemical
compositions. We see a correlation between the orbital characteristics
of the planetary companions and the metallicities of their parent
stars for the eight systems with mass functions below 13 MJup.
Here we report on spectroscopic abundance analyses of
upsilon And, tau Boo, rho^{1} Cnc, 16 Cyg B,
51 Peg, 47 UMa, 70 Vir, and
HD 114762. The "51 Peg-like" systems, upsilon And, tau Boo,
rho^{1} Cnc, and 51 Peg, are very metal-rich relative to the
Sun, with a mean [Fe/H] value near 0.25. The stars 16 Cyg B and
47 UMa, which have companion masses near 2 MJup, are only
slightly more metal-rich than the Sun. The two systems
with the highest mass functions, 70 Vir and HD 114762, also have the most
metal-poor parent stars. There is also some weak evidence that
the Sun may have been self-enriched sometime in its history. Given these
observations, we propose that the original metallicities of the parent
stars of the known planetary systems have been altered to varying
degrees by mechanisms related to planet formation.
The Visible Spectrum of Gl229B: A Window into the Deep Atmosphere of a Brown Dwarf
Caitlin A. Griffith (1), Mark Marley (2), Thomas Greathouse (1),
and Bruno B\'ezard (3)
We have observed the brown dwarf, Gl229B, at visible wavelengths
from the Keck telescope, equipped with the HIRES spectrometer.
We find that Gl229B is a ~ 17.5 magnitude object at wavelengths of
0.6-0.7 micron. This flux exceeds that possible from the reflection of
the primary M1 star's radiation off of the brown dwarf by a couple of
tens of magnitudes. In addition, the brightness of Gl229B indicates
that we are observing atmospheric levels having ambient temperatures
of 2000K, significantly deeper than the level responsible for Gl229B's 1000K
effective temperature. This window lies at spectral regions of relativley
weak water absorption; it's existence is supported by radiative
transfer models of the atmosphere.
The temperature level that we detect is significantly
deeper than that previously detected on a jovian planet.
Gl229B's visible spectrum therefore provides a unique window into the
deeper levels of jovian atmospheres, where the atmosphere's
chemistry changes from a reduced CH_4 rich atmosphere to a CO
rich atmosphere, and where previously unexplored cloud structures
(e.g. condensed silicate and iron) may reside.
Astrometric detection of a very low mass companion orbiting a late-type radio star
J.C. Guirado (1,2), J.E. Reynolds (3), J.-F. Lestrade (4),
R.A. Preston (1), D.L. Jauncey (3), D.L. Jones (1),
A.K. Tzioumis (3), R.H. Ferris (3), E.A. King (5),
J.E.J. Lovell (5), P.M. McCulloch (5), K.J. Johnston (6),
K.A. Kingham (6), J.O. Martin (6), G.L. White (7), P.A. Jones (7),
F. Arenou (4), M. Froeschlé (8), J. Kovalevsky (8),
C. Martin (8), L. Lindegren (9), S. Soderhjelm (9)
VLBI (very long baseline interferometry) astrometry,
using the phase-referencing technique, provides
submilliarcsecond-precise positions of weakly-emitting
radio stars. From a combination of multiepoch VLBI
astrometric observations and Hipparcos satellite data
we have detected a low-mass companion orbiting the
radio star AB Doradus, a member of the Pleiades
Moving Cluster. The dynamical mass of the newly-discovered
companion, inferred from the observed reflex motion of
ABDoradus, places this object on the boundary between
a brown dwarf and a very low-mass star. If accurate
photometric information can be obtained, our precise
mass estimate could serve as an accurate calibration
point for different theoretical evolutionary models
of very low-mass objects. This result is a demonstration
of the capabilities of the phase-referenced VLBI astrometry
techniques, which should be a powerful tool for detecting
brown dwarfs and planets orbiting radio stars.
The distribution of mass-ratios of late-type main sequence
spectroscopic binaries
J.L. Halbwachs (1), M. Mayor (2) and S. Udry (2)
An unbiased sample of K-dwarf stars was observed on an timespan of about 10
years in order to find spectroscopic binaries. The orbital elements of 21 SBs
with periods between 1 day and 10 years were thus obtained. These stars were
added to the G-dwarf binaries of Duquennoy and
Mayor (1991), providing an unbiased sample of 41 SBs.
The distribution of the mass-ratios of late-type dwarfs was then investigated,
including an estimation of the proportion of brown dwarfs.
Deep searches in Open Clusters
N.C.Hambly
Deep searches for brown dwarfs in open clusters provide a means for the
detection of substellar objects of known distance, age and metallicity.
A review is presented of recent work, leading to the discovery of the
first 'bona fide' free-floating brown dwarf. The direction of current
and future research in this area is discussed.
Confirming Planet Detections with Spectral Line Bisectors:
Does alpha Tau Have a Planet?
Artie P. Hatzes
Stellar radial velocity (RV) variation can arise from a number
of phenomenon: radial and nonradial pulsations, stellar surface
structure, and of course companions, so it is important
that extra-solar planets discovered with RV measurements be
confirmed with independent measurements. A lack of variability
in the spectral line shapes, as measured by the line bisector,
would be conclusive evidence of a planetary companion.
The changes in spectral line bisectors and RV amplitude are
calculated for both nonradial pulsations and cool starspots.
It shown that line bisector measurements may provide the
most sensitive tests for planet confirmation.
Several K giants show long-period RV variability which
may be due to planetary companions. The line bisectors
are examined for the K giant alpha Tau (P_{RV} = 654 m/s)
and variability is found with a period of 50 days. This is most
likely due to pulsations which suggests that the 654-day
period is due to an 11 MJup companion. At McDonald Observatory
we are currently in the process of obtaining high resolution
(R = 200,000) data on other K giants as well as solar-type stars
with purported planetary companions.
The McDonald Planet Search Programme
A. P. Hatzes, W. D. Cochran
An overview of the McDonald Observatory Planet search
program will be presented. This program
searches for planets using precise stellar radial velocity measurements
taken at the 2.7-m Harlan J. Smith telescope at McDonald Observatory.
It has experienced recent success in the discovery
of a 1.5 M_Jupiter planet in a highly eccentric orbit around
16 Cyg B. Another important aspect of this program is the
use of spectral line bisectors to confirm planet detections.
The bisector variations in 51 Peg recently reported by David
Gray are compared to those taken at McDonald Observatory and
the issue of whether our measurements contradict David Gray's result is
addressed. The future of our program, in particular the use
of the Hobby-Eberly Telescope to survey a large sample of stars,
is also discussed.
A new population of Brown Dwarfs
M.R.S. Hawkins (1), C. Ducourant (2), H.R.A. Jones (3,4),
and M. Rappaport (2)
A large area deep survey for brown dwarfs from digitally
stacked 1.2m Schmidt plates has revealed a population of very red
stars (R-I > 3). On the basis of their colours, near IR spectra
and their luminosity estimates from proper motions, these objects
appeared to be Brown Dwarfs. First measurements of the parallaxes of
8 of the reddest objects at the 1.5 danish telescope confirmed this,
and showed that four of them have K-band luminosities some mags below
the bottom of the main sequence. We present here those results.
Low Mass Characters Lurking in the Solar Neighborhood
Todd Henry
"Nearby Stars" will be discussed in the context of the sample of
stellar neighbors within 10 parsecs of the Sun. The prevalence of
companions found for all types of stars in the sample will be
reviewed, with special attention to the low-mass secondaries.
Observational techniques effective at detecting companions with
separations 0-1000 AU will be included. Characterization of the
lowest mass objects known, specifically near the stellar/substellar
break, will be discussed in detail. It will be pointed out just how
far we have yet to go in order to detect directly any planet orbiting
these stars.
The mass function into the brown dwarf regime
Hugh R.A. Jones (1,2)
In a recent survey for faint red stars from a digital stack of Schmidt
plates a number of candidate objects were identified. Parallaxes for
six of these stars have now been obtained, four of which are nearby
and from their very low luminosities seem to be true brown
dwarfs. We present various spectral analyses of these and other
brown dwarf candidates selected from the digital stack and show
how they may be used to derive the first field star mass function
into the brown dwarf regime.
Spectral evidence for dust
Hugh R.A. Jones (1,2) and Takashi Tsuji (1)
We have compared optical and infrared spectra of late-type M dwarfs
with a variety of different synthetic spectra. We find that the
match between observation and theory is much improved when comparisons
are made with models allowing for the formation of dust. We present two
compelling pieces of evidence for dust (1) a dramatic change in the spectra
of late-type M dwarfs at wavelengths shorter than 0.75 microns and (2) an
excellent fit to the spectrum of the brown dwarf candidate GD165B.
It thus seems that a substantial part of the long-standing discrepancies
between late-type M dwarf models and observations arises from the lack of
inclusion of dust in models for late-type M dwarfs as recently proposed by
Tsuji, Ohnaka & Aoki, A&A, 305, L1
Geometrical simulations of cloud fragmentation and possible implications for the IMF
A.A. Kaas (1), H. Kristen (1) and G. Östlin (2)
We present studies of simple, geometrical models of the fragmentation of
molecular clouds. Motivated by the seemingly universal mass spectrum of
molecular cloud clumps, dN/dm ~ m^{-1.6}, we have used the concept
of "domain packing", introduced in astronomy by Richtler (1994), as a method
of simulating a stochastic mass fractionation process. Our results show that
a power-law mass spectrum, dN/dm ~ m^{-2}, develops for an increasing
part of the considered mass interval as the filling factor increases. This
result is independent of the extent of the considered mass interval. Assuming
that the molecular clouds are not ideal spheres, but rather filamentary or
flattened structures, we have introduced corresponding boundary effects in the
model. As expected, the resulting mass spectra become somewhat shallower,
i.e. closer to observations, in the high mass region.
In order to approach the 'stellar' IMF, which is observed to be much
steeper than the cloud clump spectrum (at intermediate to high masses), we
have modeled subfragmentation of the cloud clumps with a set of algorithms.
The parameters have been constrained as far as possible from observations.
Subfragmentation according to our schemes has pronounced effects on the cloud
clump spectra, transforming them to IMFs with steeper high mass regions and
turnovers at a critical low mass.
A Coronagraphic Survey for Replenished Dust Disks Around Main Sequence Stars
Paul Kalas
Dust disks around main sequence stars are thought to be
continuously replenished by the collisions or sublimation
of planetesimals, and therefore constitute compelling
evidence for the formation of at least small bodies around
other stars. The thermal infrared signature of these
dust disks is called the Vega Phenomenon, after the discovery
of FIR excess around this A0V star in IRAS data.
Researchers subsequently
estimated that roughly one quarter of nearby main sequence
stars in a broad spectral range are surrounded by orbiting
dust - implying that 25% of stars possess planetesimal
systems. However, only one of these Vega-like stars, Beta
Pictoris, has an optically resolved dust disk.
Over the past three years, we endeavored to image
these replenished dust disks in the optical using
a coronagraph and the University of Hawaii 2.2 m
telescope on Mauna Kea. 106 main sequence
stars were imaged, with no disk detections among
the stars previously identified as Vega-like, including
several with FIR excesses comparable to Beta Pictoris.
Models of scattered light from
dust disks were used in data simulations to
show that dust disks similar to the one found around
Beta Pictoris would have been detected around other
stars if indeed they existed. However, we did find
reflection nebulosity around two Vega-like stars,
HR 241 and HR 1307,
but with a morphology resembling
that of the well-known Pleiades reflection
nebulosity. From these results we conclude that:
1) The detection of a large FIR excess in main
sequence stars is not well-correlated to disk-like
reflection nebulosity, such as the one found around
Beta Pictoris.
2) The extended, disk-like morphology of dust seen
around Beta Pictoris is unusual among a sample of
stars with comparable excess FIR emission. We speculate
that the possible gravitational perturber responsible
for the known disk asymmetries may also be responsible
for the extended distribution of dust.
3) The number of stars estimated from the IRAS data
as having replenished dust has been overestimated by
as much as 50% for early spectral types. Many stars displaying the Vega
Phenomenon are in fact heating the ISM, and some produce
reflection nebulosity resembling the Pleiades. Thus,
the fraction of planetesimal systems around nearby stars,
as deduced from the FIR signature of replenished dust,
is closer to 10%.
Spectroscopic Properties of Ultra-cool Dwarfs and Brown Dwarfs:
Current Understanding and Lingering Puzzles
Davy Kirkpatrick
This talk will highlight the optical and near-infrared spectroscopic
properties of ultra-cool dwarfs (defined as spectral types >= M7),
known brown dwarfs, and brown dwarf suspects.
Only targets successfully imaged will be discussed,
and these include isolated field dwarfs, wider companions to known
nearby stars, and low luminosity objects in clusters.
First, the development of a taxonomy for such objects over a broad
range in wavelength, including the placement of such
objects like GD 165B and Gl 229B in the extended M dwarf spectroscopic
sequence, will be addressed.
Second, the use of specific lines and molecular features as temperature
and luminosity indicators will be explored
along with a review of results on the ``lithium test'' for both cluster
members and field objects. Last, some lingering questions will
also be asked. Among them are (1) Where are objects which
bridge the gap between very red dwarfs like GD 165B (J-K ~ 1.8)
and brown dwarfs like Gl 229B (J-K ~ 0.0) and how do we
uncover examples?, (2) How will grain formation affect the appearance
of spectra and for what temperatures are these effects
important?, and (3) Why does the near-infrared spectrum
of GD 165B fit in so nicely with other late M dwarfs while its optical
spectrum bears little resemblance to any other known object?
The Stellar Mass Function
Pavel Kroupa
I begin with an outline of the tremendous progress achieved during
the past 15 years in constraining the faint-star luminosity function
(LF) using different observational approaches. These include counting
stars within a distance of 5-20 pc from the Sun, photographic and CCD
star counts extending to distances of 100-200 pc, counting faint red
dwarfs seen projected against dark background molecular clouds, and
star-counts with the Hubble Space Telescope (HST) reaching to
distances of a few hundred pc for the faintest M-dwarfs.
The photographic, CCD and HST (i.e. deep pencil-beam) star-counts
along different lines-of-sight into the Galactic disk yield
essentially the same LFs. These, however, are significantly deficient
in stars fainter than M_{V} ~ 11 when compared with the
nearby LF - a difference that has been much debated during the past
10 years.
Taking the stellar velocity dispersion in the Galactic disk to be
about 30 km/s, essentially complete exchange of stars inside a
sphere of radius 3 kpc occurs over a time-span of 10^{8} yr. This
explains the agreement between the deep LFs. That the solar
neighbourhood could truly be overdense in low-mass stars can be
excluded by the same argument. The apparent discrepancy between the
nearby and deep LFs is resolved naturally by taking into account that
50-60per cent of the `stars' in the Galactic disk are multiple
stellar systems which cannot be resolved in the deep surveys.
To extract the mass function (MF) from the LF, the slope of the
stellar mass-(absolute-magnitude) relation is needed. It is well
known that this relation is rather uncertain for stars fainter than
about M_{V}=11, where stars become fully convective and H_{2}
formation in the outer stellar region has an increasingly significant
effect on the properties of a star. Recent theoretical progress in
the treatment of stellar structure and atmospheres, and improved
observational constraints of the mass-(absolute magnitude) relation,
show that this slope has a pronounced minimum near M_{ V}=11.5,
which is why the LF has a pronounced maximum at this magnitude.
Modern stellar models show good agreement with the location of the
`H_{2}-convection maximum' in the LF.
While the nearby LF is based on a stellar census in which most faint
companion stars have been found, constraints on the MF are poor
because the sample is small. In order to estimate the MF from the much
better defined deep LFs the mass-ratio distribution of binary systems
must be known reasonably well. This has been achieved through
long-term radial velocity surveys of nearby stars.
It follows that the MF can be approximated by Salpeter's MF in the
stellar mass range 1-0.5 Msun. For 0.5-0.08 Msun the
power-law index is somewhat less steep. This two-part power-law
approximation for the MF of stars less massive than 1 Msun is
also consistent with microlensing data towards the Galactic Bulge.
The contribution by main-sequence stars to the local dynamical
Galactic disk surface mass density,
Sigma_{dyn} = 48+-9 Msun pc^{-2},
is Sigma_{st} ~ 30+-8 Msun pc^{-2}.
The difference between the dynamical mass
and the stellar mass can be accounted for by gas, dust and stellar
remnants. No other dark matter is needed locally.
I also discuss the stellar samples in the Hyades, Pleiades and
Trapezium clusters. Presently there is no evidence for different
initial MFs among different populations of stars in the Galactic
disk. Finally, I suggest future work that would improve the present
constraints on the MF.
On Possible Multiplicity of Components of some Low Mass Systems
D. A. Kovaleva
An attempt is made in this work to explain a shift of low mass
components of binaries from their "normal" positions in the
mass-luminosity plane. The assumption that "overmassive" stars are
unresolved binaries (multiples) was investigated.
The technique of the optimal splitting of these objects into the
components located as near as possible to the MLR (whithin
observational errors) has been designed. The coordinate descent
algorithm was used for optimization of component parameters. We
calculate the most probable mass and luminosity values of hidden
components.
Some observational parameters are also predicted for the resolved
components. There are reasons to expect that some brown dwarfs could
happen among these supposed unseen components of low-mass stars.
Low-Mass Companions Found in a Large Radial-Velocity Survey
David W. Latham (1), Robert P. Stefanik (1), Tsevi Mazeh (2),
Guillermo Torres (1), and Bruce W. Carney (3)
In 1983 we undertook a survey of a proper-motion-selected sample of
solar-type stars to study the kinematics
and chemistry of the halo and disk populations in our Galaxy (Carney &
Latham 1987, AJ, 93, 116; plus 12 subsequent papers in the AJ). Over
the intervening years we have continued to monitor the radial
velocities of more than 1400 stars in this sample, to determine the
frequency and orbital characteristics of the spectroscopic binaries.
In this paper we present a status report on our results for the
low-mass companions found in the survey, based on orbital solutions
for more than 150 single-lined binaries. We also present the
eccentricity distribution of our sample versus period and versus
minimum companion mass.
ISO Observations of Pleiades Brown Dwarfs
Leech, K., Claes, P., Laureijs, R., Martin, E.L., Metcalfe, L., Prusti, T.,
Salama, A., Siebenmorgen, R., Trams, N., Zapatero-Osorio, M.R.
Teide 1 and Calar 3 are unique objects in being the first free-floating Brown
Dwarfs discovered. This point deserves stressing - they are not companions to
other objects (and in fact are distant from other objects) and are certainly
Brown Dwarfs. IR observations will provide important inputs to models of Brown
Dwarf atmospheres. As unique objects they deserve intensive study to more fully
characterise the poorly known but potentially very important class of objects.
The Infra-Red Space Observatory (ISO) Observing Time Allocation Committee
accepted a proposal to try and image one of the Pleiades Brown Dwarfs with the
ISOCAM instrument through two filters. This poster discusses the near-IR flux
expected from the Brown Dwarfs and the expected instrumental setup required to
detect them.
The low-mass companions in the nearby triple system LHS 1070
Christoph Leinert (1), France Allard (2), and Andrea Richichi(3)
Infrared speckle observations had shown that the nearby M5.5 dwarf
LHS 1070 has two companions at ~ 1", separated among them
by ~ 0.4". Based on the absolute K magnitudes of 10.5 mag
and 10.8 mag for the components, we estimated the masses of components
B and C to be close to the hydrogen burning mass limit of 0.08 Msun
(Leinert et al., A&A Letters 291, L47, 1994).
We now present BVRIJHK photometry of the
components and optical spectroscopy between 400 nm and 850 nm,
obtained partly from ground, partly with the WFPC and FOS instruments
on HST. A comparison to plane parallel model atmospheres
(Allard and Hauschildt 1997)
gives first results on effective temperatures. The best fitting values
are 2900 K for component A and 2700 K for components B and C.
These values are considered as upper limits until questions of
grain condensation in cool dwarf atmospheres have been settled.
With the absolute V magnitudes then masses for
components B and C of M_B = 0.085 Msun and
M_C = 0.088 Msun are derived from the 10 Gyr solar
metallicity isochrones in Baraffe et al. (1997).
The orbital motion of components B and C now has been followed
over an arc of ~ 45 deg. The relative postitions
so far indicate a mass sum of these components of roughly
0.17 Msun. A significant increase in accuracy of
mass determination is expected by following the relative
motion over a few more years.
The present conclusion on the companions to LHS 1070 is:
they probably are M dwarfs with masses close to the
hydrogen burning cutoff.
The Low-Mass IMF in L1495E
Kevin Luhman and George Rieke
To measure the low-mass IMF in a nearby region of star formation, we have
obtained IR imaging and spectroscopy of sources in the
L1495E cloud within the Taurus-Auriga complex.
We have completed IR spectroscopy on the X-ray selected
sample of Strom & Strom (1994) and used IR spectral types to place the sources
on the H-R diagram. Using the
latest theoretical evolutionary tracks, we find masses down to 0.1 Msun
in this spectroscopic sample. With the star formation history revealed in
this H-R diagram as in input to IR luminosity function modeling, we derive the
IMF into the brown dwarf regime and estimate the completeness of the X-ray
sample.
A Deep Glance at the Praesepe Open Cluster
Antonio Magazzù
We present the results of a deep CCD survey of an area of about 800 square
arcminutes in the Praesepe open cluster. We have selected a faint, red object
with R ~ 21, R - I ~ 2.8 as a substellar candidate. Follow up infrared
photometry and low-resolution spectroscopy in the range 640-920 nm confirm
that our object is a very late M type dwarf. This is the faintest and coolest
object in Praesepe whose spectrum has been obtained to date. Its colours and its
spectrum are consistent with a brown dwarf of the age of the cluster. Our
findings are compared with recent results in the Pleiades.
The IMF of Low Mass Stars in Globular Clusters
Guido De Marchi, and Francesco Paresce
We compare deep I-band luminosity functions of two globular clusters
(NGC 6397 and NGC 6656) that have similar metal content, but have
experienced very different dynamical histories, both in terms of
internal dynamics and as a result of different interactions with the
Galactic tidal field. Their luminosity functions measured at the
half-mass radius are indistinguishable from one another within the
statistical errors over a range of more than 5 mag. They both show an
increase with decreasing luminosity up to a peak at M_I ~= 8.5,
and then drop all the way to the detection limit. We use the presently
available theoretical mass-luminosity relations to convert these
luminosity functions into mass functions, and show that they all share
one basic feature, namely an exponential rise with decreasing mass down
to about 0.2 solar masses followed by a plateau. We conclude that an
IMF that flattens out and possibly drops below ~ 0.2 solar masses is
the only viable hypothesis for the low metallicity globular clusters
studied so far.
New Extrasolar Planets
Geoffrey W. Marcy (1,2), and R. Paul Butler (1,2)
Highly precise Doppler measurements, with errors less than 15 m/s, of 260
Solar-type stars have revealed 7 companions that have low masses, Msini =
0.45 - 7 MJup. Approximately 5% of Solar-Type stars have
such companions within 2.5 AU. These companions exhibit a mass distribution
that peaks at about 1 MJup, and perhaps lower. Thus, they
appear to represent the high-mass tip of the planet mass function.
Companions having masses less than 1 MSat are
currently undetectable as are planetary companions with orbital periods
longer than 9 yr.
Among the 8 lowest-mass companions detected by Doppler measurements, three
preliminary ``classes'' are emerging. The ``51 Peg'' class contains 4
representatives, having small semimajor axes, a < 0.15 AU and circular
orbits. Their low eccentricities appear to be primordial rather than
tidally induced. A second class of ``eccentric'' planets contains 3
representatives, all having e > 0.35 and Msini > 1 MJup.
Finally one planet (around 47 UMa) has a circular orbit, a = 2.1 AU, and
Msini = 2.4 MJup, characteristics not qualitatively distinctive
from Jupiter in our Solar System.
Modifications to standard planet-formation theory are emerging to account
for the 3 classes of planets. Protoplanetary disks may span a range of mass
densities, external environments, and metallicities that yield a
variety of planet-formation scenarios. The G2 binary twins 16 Cyg A & B
differ in that component B has a planet but only A shows lithium. Both facts
arguably stem from differences in their original protostellar disks.
The 51 Peg planets may result from viscous loss of orbital angular momentum
in particularly massive disks. The eccentric planets
can result from gravitational interactions of multiple
planets (originally in circular orbits) or from resonances
in the disk that feedback gravitationally on the
protoplanet. Those disk conditions that are required for
the formation of true Solar-System analogs remain
unknown, as is the occurrence rate of such conditions.
New techniques, such as Keck Doppler measurements, astrometry, and
ground-based interferometry, will detect Saturn and Neptune-like planets.
A balloon-borne telescope equipped with an apodized mask, Lyot stop, and
adaptive optics may cheaply and quickly make direct detections of planets.
Atmospheres of Giant Planets from Neptune to Gliese 229B
M.S. Marley
The diverse and complex atmospheres of the solar jovian planets provide
substantial insight into the processes that govern the atmospheres
of extrasolar giant planets and the cooler brown dwarfs. Indeed
Jupiter, Saturn, Uranus, and Neptune provide
ample warning that the atmospheres of the extrasolar jovian planets
will likely depart substantially from the modelers'
ideal, spatially-homogeneous, solar-composition constructs.
In this review I will discuss the lessons learned from the exploration
and modeling of the solar jovian atmospheres. I will discuss processes,
like condensation and photochemistry, that substantially influence the spectra
and thermal structure of these atmospheres yet are very difficult to
model on an a priori basis.
I will then present our own atmosphere models
for the extrasolar giant planets and Gliese 229 B.
We find that the 4 to s5 micron window in NH3, CH4 and H2O opacity
is open for objects with effective temperatures from 100 to 1000 K.
Thus as is the case for Jupiter and Gl 229 B, the emitted flux in this
spectral region is significantly greater than the blackbody flux for
the planetary effective temperature. For objects with Teff > 500 K,
the flux in regions of low CH4, H2O, and H2 - H_2 opacity within
J, H and K bands also substantially exceeds the blackbody flux, as
in the spectrum of Gl 229 B. Depending on the
individual planet, the reflected flux in the near-infrared begins to
exceed the thermal flux for objects cooler than about 400 K.
Five of the extrasolar giant planets
(70 Vir b, 47 UMa b, Gl 411 b, 55 Cnc c, and HD 114762 b)
have estimated effective temperatures below ~ 800 K.
Water clouds should be present in all these atmospheres except for
70 Vir b and HD 114762 b. Water marginally condenses in
the atmosphere of the former while that of the latter should be essentially
cloud free.
The presence or absence of clouds dramatically affects the reflected
component of the near-infrared spectrum. In objects without clouds,
most of the incident radiation beyond one micron
is absorbed before it can be scattered. In somewhat cooler objects,
a water cloud near the tropopause can increase the reflected flux
by up to two orders of magnitude in this spectral region. As has been
the case with Jupiter and the other solar jovian planets, the
near-infrared spectra of extrasolar jovian planets will reveal much about
their vertical cloud and haze structure.
The Brown Dwarf Mass Function in the Pleiades
Eduardo L. Martín, M.R. Zapatero Osorio, R. Rebolo
The Pleiades open cluster is the location where the first confirmed
free-floating brown dwarfs have been discovered (Rebolo et al. 1996,
ApJ, 469, L53).
New searches are revealing an increasing substellar population in the cluster
(see abstracts by Cossburn, Festin, Schultz, and Zapatero-Osorio).
We review the results of several surveys covering different areas
and with varying completeness magnitudes. About 20% of the cluster
area has been surveyed with sensitivity up to 30 Jupiter masses.
Thus, the Pleiades is rapidly becoming the first place where
it is possible to derive a statistically significative mass
function for isolated brown dwarfs.
We discuss in detail the uncertainties involved in converting the results of
photometric surveys to luminosity and mass functions.
We show that although the uncertainties are still high, it is likely
that the mass function does not have a sharp turnover at the substellar
limit. The possibility of a cutoff in the mass function below 40 Jupiter
masses is discussed.
Orbital properties of substellar objects around stars
M. Mayor, D. Queloz and S. Udry
Radial velocity surveys have revealed a significant number of very
low-mass companions to solar-type stars. The fraction of secondaries
with M_2sini below the Hydrogen-burning limit can not only be
explained by the sini distribution. A few percents of solar-type
stars have a companion in the mass range of brown dwarfs.
The mass distribution of stellar secondaries seems to have a
continuous tail down to the domain of heavy planets. Despite the small
size of the statistics, the possibility is offered to compare orbital
properties as function of the secondary masses. The transition region
from brown dwarfs to heavy planets is of special interest.
Brown Dwarf Companions to Pre-Main Sequence Stars:
Effects on Circumstellar Disk Evolution
Michael R. Meyer
Considerable attention has been given recently to the effect of
companion stars on the evolution of circumstellar disks.
Jenson et al. (1994) suggest that the outer disks of T Tauri
stars are affected by the presence of a binary companion with
projected separations 1-50 AU. However, Simon and Prato (1995)
find that the properties of inner disks (0.1-1.0 AU) are not
affected by companions at similar projected separations.
We report new mid-infrared observations of two young stars
found in the Taurus dark cloud; the weak-emission T
Tauri DI Tau as well as the nearby classical T Tauri star DH Tau.
DI Tau, tentatively identified by Skrutskie et al. (1990)
as an object in the process of
dissipating its circumstellar disk based on IRAS observations,
is found to have no infrared excess at a wavelength of 10 microns.
DH Tau exhibits excess emission at 10 microns consistent with
predictions based on circumstellar disk models. While both
objects appear to have the same stellar mass, age, and
rotation rate, they differ in two fundamental respects;
DH Tau is a single star with an active accretion disk
and DI Tau is a binary star lacking such a disk. The companion
has a very low luminosity and is located
at a projected distance of 19.2 AU from
the primary. Adopting a mass-luminosity relationship appropriate
for the evolutionary state of the system suggests that the
companion has a mass well below the hydrogen burning limit.
We speculate that the formation of a sub-stellar mass companion
has led to the rapid dissipation of the circumstellar disk
that recently surrounded DI Tau. We discuss these results in the
context of; i) the frequency of brown dwarf companions to PMS stars;
and ii) the evolution of circumstellar disks from optically-thick
to optically-thin in the presence of a binary companion.
Secondary Mass Distribution for Low-Mass Companions Found
in a Large Radial-Velocity Survey
Tsevi Mazeh (1), Dorit Goldberg (1), and David W. Latham (2)
We present an analysis of the distribution of secondary masses for the
binaries found in the Carney-Latham proper-motion sample. We show that
the secondary mass distribution rises mildly towards the substellar limit.
We comment on possible parameters of the binary population on which the
secondary masses might depend.
Molecular Hydrogen Parameters for Brown Dwarfs and Extrasolar Planets
Michael E. Mickelson
State-of-the-art high resolution spectroscopy of
cosmic sources has driven the need in recent years
for new high resolution laboratory measurements
and improved theories of atoms and molecules.
Molecular hydrogen is the major gaseous constituent
in the atmospheres of the outer planets and plays
an important role in radiative transfer through
its prominent spectrum and its effects on other
atomic and molecular species. The hydrogen continuum
and dipole spectrum produced by various collisional
and intermolecular interactions must be directly
factored into the radiative transfer in atmospheric
models, whereas the sharp well defined quadrupole
lines can be used as sensitive probes of atmospheric
conditions.
This paper presents a compilation of the most current
values of the ground state molecular parameters for
the hydrogen molecule. Using the "zero density"
line positions for hydrogen quadrupole lines, a
self consistent set of molecular constants have
been determined by fitting all known lines from
the 0-0 through 5-0 bands. Also included are pressure
shift and pressure broadening coefficients, line
strengths, as well as documentation of Dicke narrowing
and the Galatry line shape.
This paper provides in one place, all the most
important ground state parameters for hydrogen quadrupole
spectra and should be of significant help in modeling
and probing the atmospheres of Brown Dwarfs and
Extrasolar Planets.
Adaptive Optics Coronagraphy for Brown Dwarf and Jovian Planet Detection
Tadashi Nakajima
For last five years, there has been a collaborative effort among
astronomers at Caltech and the Johns Hopkins University in search
of brown dwarf companions around nearby stars with an adaptive optics
coronagraph operated at optical wavelengths. For last two years the search
wavelengths were extended to the near infrared. We summarize
the detection statistics of stellar/substellar companions.
Then we describe in detail about our best success so far, the detection of
the cool brown dwarf, Gliese 229B, and its follow-up observations
and interpretations.
Finally, a brief description of what I personally believe is a viable option
for extra-solar Jovian planet detection using adaptive optics coronagraphy
with a large ground-based telescope is presented.
Direct imaging of extra-solar planets
Scot S. Olivier, Claire E. Max, Jim M. Brase, Donald T. Gavel,
and Bruce Macintosh
Direct imaging will allow physical characterization
of extra-solar planets on several levels of
sophistication. The broad-band color of the planet
together with its mass and distance from the
central star can yield information about the
planet's temperature and albedo. With the help
of simple modeling, a planet's color allows its
differentiation as a gas giant or a rocky planet.
On a more advanced level, broad-band spectroscopy
could allow measurement of atmospheric constituents
that can reveal detailed physical characteristics
of extra-solar planets, and could even indicate
the presence of life.
The technical challenge for the direct imaging of
extra-solar planets lies in separating the
planet's light from that of its parent star. In
this paper, we will discuss the direct imaging
capability of the new generation of large ground-
based telescopes equipped with adaptive optics
systems. Scaling laws for the performance of
these systems as a function of telescope diameter
and number of adaptive optics elements will be
derived and compared to the results of simulations
and data.
ISOCAM observations of pre-brown-dwarfs (?)
Olofsson, G. et al.
According to current model calculations, brown dwarfs are relatively bright
during their contraction phase. Thus, for a mass of only 0.01 Msun the
predicted luminosity is about 0.01 Lsun at an age of 10e^{5} years ,
which should be easily observable for the closest star formation regions.
The temperature for such a source decreases slowly and remains above 2000K
at an age of 10e^{6} years and should thus be observable in the near infrared
with present sensitive arrays. If, however, the source is deeply embedded in
the parental cloud and/or obscured by circumstellar matter it may be too
faint to be seen, at least in the J filter. This will make it difficult to
identify the source as a young stellar object and in addition the luminosity
correction due to the extinction will be uncertain. Therefore the NIR data
needs to be complemented with observations at longer wavelengths and ISOCAM
offers the required wavelength coverage and sensitivity for such an investigation.
We are currently conducting an ISOCAM survey programme of nearby star formation
regions using two broad filters centred at 6.75 and 15 microns. A quite clear
distinction is seen in the colour index [15/6.75] between young stellar objects
(YSOs) and background stars (see Nordh et al.,1996,A&A 315, L185). In the
Chameleon I cloud (for which the data reduction so far has advanced the furthest)
most YSOs exhibit T Tauri-type spectral energy distributions and assuming that
the fainter members do not differ outside the observed wavelength range from
the brighter ones, we can derive a luminosity function. If we then assume that
the median age for the well observed brighter YSOs also can be applied to the
fainter members, the masses can be deduced. We find that a large fraction of
these objects will end up as brown dwarfs. It must be emphasized, however, that
our analyses is as yet very preliminary.
Palomar Search for Companion Brown Dwarfs: Gliese 229B
Ben R. Oppenheimer (1), Tadashi Nakajima (1), Shri Kulkarni (1),
Keith Matthews (1), and David Golimowski (2)
At Palomar Observatory we have conducted an optical coronagraphic
and infrared direct imaging survey of over 350 nearby stars to find
faint companions of these stars. A number of candidate companions
have been identified and await confirmation through further
observations. We have confirmed several of these candidates,
including two late M dwarfs and one brown dwarf, Gliese 229B. To date
we have observed about 60% of the volume-limited 8pc sample,
consisting of 185 stars. We present a discussion of the statistical
properties of this sample, including what conclusions can be drawn.
Since the discovery of Gliese 229B, we have obtained detailed
observations of it spanning the wavelength range from 0.85 microns to
10.5 microns, including photometric measurements and spectroscopy.
These data were collected using the 60 inch and 200 inch telescopes on
Mt. Palomar, Keck, HST, and UKIRT. We review this data to provide a
comprehensive picture of a cool brown dwarf.
Recent Measurements of Stars with Planets
using the Palomar Testbed Interferometer
Xiaopei Pan (1), Shri Kulkarni (1), M. Mark Colavita (2), Michael Shao (2)
The recent discoveries of extra-solar planets has generated great
excitement, and have dramatically changed our view of the
universe. However, some important questions which have been raised
include: what is the physical environment of those star-planet
systems? Are those stars with ``Hot Jupiters'' really like the Sun ?
The Palomar Testbed Interferometer (PTI), a long-baseline IR
interferometer at Palomar Observatory, reaches a resolution of 2 mas,
and is capable of resolving the disks of many nearby stars and
determining their fundamental parameters. For example, Lalande 21185,
which is claimed to have two long period planets, has had its diameter
determined with PTI. This is the first direct measurement of the
diameter of an M-dwarf main-sequence star, and provides important
clues to the physical characteristics of the M dwarf type of star.
Another star, 51 Peg, around which the first extra-solar planet was
discovered by radial velocity measurements, is considered like the
Sun, and is classified as a G2 main-sequence star. Since its distance
has been determined accurately by HIPPARCOS as 15.36 +- 0.01 pc, the
theoretical estimate of its angular diameter is about 0.6 mas, which
should not be resolved by PTI. However, preliminary results from PTI
indicate that the disk of the star has been resolved, and that its
physical size is much larger than the estimated value.
This paper will summarize recent measurements of the planetary stars
with PTI, and analyze the physical characteristics of those interesting
targets.
IR Interferometry and its Application to the Search for Brown Dwarfs
and Extrasolar Planets
Francesco Paresce
The coherent combination of light from a synthetic aperture array
of several large diameter telescopes offers the possibility of
attaining the ultimate achievable angular resolution and sensitivity.
This combination of characteristics promises to revolutionize
the way we observe all astronomical sources but especially the faint
brown dwarfs and extrasolar planets we might expect from recent
theories and discoveries. A number of facilities are being built or
planned for operation in the near future that meet the stiff requirements
associated with high precision measurements of these objects. The most
promising of these will operate primarily in the IR regime in order to
exploit the particularly favorable observing conditions in this range.
I will use the example of the ESO VLT Interferometer that is currently in
an advanced phase of construction to illustrate with practical examples
the advantages and drawbacks of the technique when applied to the
observation of faint objects located close to very bright ones. I will
discuss in detail the expected scientific benefits of these facilities
and the technical challenges that will have to be met in the next few
years for this potential to become reality.
The lithium lines formation in brown dwarf atmospheres: molecules,
chromospheres, NLTE
Yakiv V. Pavlenko
Results of the study of lithium lines formation in spectra
brown dwarf candidates as well as of the coolest PMS stars
are discussed.
The investigation has been carried out in the frame of the project
aimed the realization of the "lithium test" proposed by
Rebolo et al.(1992).
LBL and JOLA models of molecular absorption were used to compute
theoretical spectra containing Li I resonance 6708 AA and
subordinate 8126 A. These two approaches give similar
(at least qualitatively) results in the wide range of Teff.
For young PMS dwarfs UX Tau C (M6, 3100/4.5) and HHJ430 (M5, 3300/4.5)
we found log N(Li) = 3.2 using both Li I lines.
From the comparison of observed and computed spectra of Pleiades brown
dwarfs Teide 1 and Calar 3 (M8, Teff ~ 2700 K) we found:
The possible impact of cromospheric-like features on
spectra of the latest M-dwarfs were studied.
NLTE computations do not give any emission in the Li line cores
for the CLF with weak and moderate gradients of temperature G_{r}.
Still common shape of spectra around Li I 6708 A line depends on G_{r}.
Namely, the flux governed by a molecular absorption in 6708 A region
increases with a grow of G_{r}.
The Darwin Infrared Space Interferometer
A. Penny (1),
C. Eiroa (2),
M. Fridlund (3),
T. de Grauuw (4),
A. Leger (5),
J.-M. Mariotti (6),
C. Schalinski (7), and
J. Schneider (6)
The Darwin infrared space interferometer is one of two candidates
for ESA's Cornerstone interferometry mission. The original
Darwin proposal to ESA's Horizon 2000+ Survey Committee envisaged an
interferometer stationed some 4 AU from the Sun with the individual
telescopes of 1.5m diameter. It was conceived to be solely intended
for the detection of Earth-sized planets orbiting nearby stars
and characterization of their spectra, with the aim of searching
for the signatures of life.
After selection by ESA as a candidate, Darwin has evolved into a more
detailed study. Options being investigated are having the telescopes
based on free-flying spacecraft with separations up to 1-200 metres.
Technical aspects of the design, and the choice of target stars, and
the prospects for use in general astrophysics, will be described.
An option for this Cornerstone mission is that Darwin would be
a moderate sized interferometer, with the main aim of doing
general astrophysics. It would be an integral structure, and
placed at the L2 Earth-Sun position.
The Mass of The Pleiades
D.J. Pinfield, R.F. Jameson, and S.T. Hodgkin
A membership catalogue for the Pleiades is
divided into four mass bins, and a tidally
truncated King profile is fitted to each bin
with good agreement to the data.
The tidal radius of the cluster is
found to be 13.1pc, and the total mass of the
cluster down to the stellar limit is calculated
to be 735 Msum.
The spread of stars in each bin, as well as
the relaxation and crossing times show the
Pleiades to be an approximately relaxed cluster
with equilibrium between the density and velocity
distributions. The cluster kinetic energy and
binding energy are consistent with the
virial theorem indicating no large unseen
population of brown dwarfs. However, the
1sigma errors in the cluster parameters provide
an upper limit to the mass of any brown dwarf
population of 131Msum, which would show up in
deep CCD surveys as <5.5 brown dwarfs per 10x10
arc-minute field in the cluster centre.
The OHP extrasolar planetary search with ELODIE
D. Queloz, J. Beuzit, J.M. Mariotti, M Mayor, C. Perrier, J.P. Sivan
A new high precision radial velocity survey is
conducted at the Haute Provence Observatory with
ELODIE. A sample of 400 stars is monitored. 25%
of the 193 cm telescope is dedicated to this projet.
Many improvements have been realised on ELODIE in
order to increase the accuracy. In particular
two scramblers have been installed.
The first results are presented.
Searching for New Brown Dwarf Candidates in a Pleiades IJK Imaging Survey
Greg Schultz
Our imaging survey for very low mass (VLM) stars and brown dwarfs covers
greater than 1200 square arcminutes in the Pleiades, and reaches estimated
limiting magnitudes of I~20, J~18.5, and K-short~17.5. Theoretical models
suggest that such depths should allow sampling of masses into the high end
of the brown dwarf (BD) mass regime, perhaps down to ~0.04 solar masses.
Six large mosaiced fields were imaged at Lick Observatory, with UCLA's
2-channel infrared camera on the 3-m telescope, and with a CCD on the
1-m telescope. Full analysis and results from at least two of these big
fields will be presented.
The largest of the mosaics covers an area approximately 19' x 19', which
alone comprises one of the largest Pleiades areas yet imaged at infrared
wavelengths. For comparison in the infrared, there have been the studies
of Williams et al (1996, ApJ, 464, 238) and Simons & Becklin (1992, ApJ,
390, 431), which imaged ~400 and ~200 square arcminutes, respectively, of
the Pleiades at K. Larger Pleiades surveys have been carried out at optical
wavelengths and have revealed brown dwarfs such as Teide 1 and Calar 3
(see e.g. Rebolo et al, 1996, ApJL, 469, L53). However, since brown dwarf
emission is stronger in the near infrared than in the optical, a principal
advantage of a survey such as ours is that it samples deeper into the BD
mass regime.
Despite the improved sensitivity to lower-mass brown dwarfs, results from
our survey so far suggest that sub-stellar objects are not turning up in
increasing numbers, at least in our Pleiades fields. But based on useful
diagnostics such as color-magnitude and color-color diagrams (e.g. I vs.
I-K, and I-J vs. I-K), some interesting new VLM/BD candidates have been
uncovered, and a few have had follow-up observations on the Keck 10-m
telescope. The latest results on these candidates will be announced and
discussed.
Treating molecular Non-LTE in cool atmospheres
Andreas Schweitzer (1), Peter H. Hauschildt (2), and France Allard (3)
Due to the low electron temperatures in the atmospheres of M and Brown Dwarfs
the common assumption of LTE could be invalid because collisional
rates might be much smaller than the radiative rates.
It has been shown that Non-LTE effects of atomic lines are small
but noticiable in the spectra of M Dwarfs.
However, Non-LTE effects of 'molecules' in cool and sub-stellar objects
are potentially much larger and can have significant observational
impact,
since their atmospheres as well as their spectra are dominated by molecular
lines.
The large number of molecular levels and lines requires special
computational method for molecular Non-LTE because of the enormous
computing requirements involved.
We present a method
(based on the Superlevel formalism)
to treat huge numbers of molecular Non-LTE levels with good accuracy.
Our method treats each line individually. However, in order to solve
the rate equations, we make the Superlevel approximation and group
energetically similar levels together to reduce the size of the
system for the levels by a factor of ~ 100,
resulting is substantial
savings in computer time and memory.
This method allows us to calculate Non-LTE model molecules with
a large number of actual levels with the existing
operator splitting and rate operator techniques implemented in the stellar
atmosphere code 'phoenix'.
Calibration of PMS Isochrones and Conversions from the
Theoretical Plane to the Observational Plane
John R. Stauffer
As photometric and proper motion surveys of star-forming regions are
pushed to fainter and fainter levels in order to constrain better the
IMF in clusters and to search for sub-stellar objects, it becomes
increasingly important to worry about which evolutionary tracks to
use and how one converts from the Log L, Log T(eff) plane to the
observable plane (or vice versa). Since no PMS star has a fundamentally
determined mass as yet (with possibly one exception), and one cannot
empirically exclude mass dependent formation epochs for stars within
a given star-forming region, it is not possible to directly and
unambiguously use the stars within very young clusters/associations
to ``validate" a given set of tracks or color-temperature conversions.
These problems are, of course, worst for the lowest mass stars because
the molecules in their atmospheres make both the calculation of the
theoretical models and the derivation of color-temperature conversions
most difficult. In this poster, I will use observational data for the
Pleiades, Hyades and other open clusters to compare to a variety of
PMS models, using several possible color or spectral type to temperature
conversions. While there is no guarantee that a set of isochrones
so ``validated" for these open cluster stars will also be ``valid" for
~ 1 Myr old clusters, some attempt at calibration (or validation)
would seem to be better than none.
Infared Spectroscopy of Low Mass stars and Brown Dwarfs
I. A. Steele (1), N. C. Hambly (2), M. R. Zapatero Osorio (3) and
R. F. Jameson (4)
We present K band CGS4 spectra of a sample of low mass stars
both in the field and in the Pleiades star cluster ranging from
M3 to M9.5. The lowest mass object for which we have data is the brown
dwarf Teide 1. For this object the 2.1~micron spectral slope and 2.3 micron
CO band strength both indicate a spectral type of M8 - M9, confirming that
deduced from optical spectroscopy.
Coronagraphic imager with adaptive optics for the Subaru Telescope
Motohide Tamura and CIAO development group
CIAO is a stellar coronagraph imager now under development for use on the
Subaru 8.2 meter telescope. The purpose of this instrument is to obtain
diffraction limited (0.05 arcsec at 2 micron) images of faint objects in
close vicinity of bright objects, which includes brown dwarfs and extrasolar
planets. For achieving both high spatial resolution and high dynamic range,
the instrument is used with the Subaru Cassegrain adaptive optics and
designed to have a sophisticated coronagraphic capability.
CIAO is optimized for use at near-infrared wavelengths (1 - 5 micron) where
the adaptive optics works most efficiently and the relative effect of
scattering by telescope and instrument optics is smaller. There are three
optical modes (i.e., high resolution mode - 0.012 arcsec/pixel, medium
resolution mode - 0.024 arcsec/pixel, and low resolution mode - 0.19
arcsec/pixel) for observations and a pupil imaging mode for optical alignments,
all of which are best optimized at J and K bands. The optics elements are
all refractive and composed of smallest number of surfaces as possible for
reducing the scattering effect. The size and shape of the occulting masks
and Lyot apodizing stops are determined from extensive computer simulations.
A number of sizes and shapes of these components are selective. Not only the
standard broad band (ZJHKLM) imaging, but also a number of narrow band
imaging as well as slit spectroscopy with grizm (R = 300 - 1200) is
supported with/without coronagraph. Polarimetry is also available for all
observing modes. CIAO will employ a 1024x1024 InSb array as a detector.
Great care is taken to design the "tension-strap supported" cryostat which
minimizes the flexure within the cryostat because the optical wave-front
sensor for the adaptive optics is attached to the outside of the cryostat.
Performance of CIAO is also extensively evaluated with computer simulations
which take into account both primary mirror errors and atmospheric turbulence.
CIAO will not only be able to employ much smaller occulting masks but also
to achieve higher dynamic range than the previous stellar coronagraph
instruments. This capability will make CIAO a unique instrument for a study
of companion brown dwarfs and extrasolar planets with various ages around
stars nearby and in star-forming regions.
The Kinematics of the Lowest Mass Stars - Are They Brown Dwarfs
Chris Tinney (1), Neill Reid (2), and Jeremy Mould (3)
The kinematics of the lowest mass field stars have been studied using
high resolution spectroscopy on the ESO 3.6m and Palomar 5m telescopes
to obtain radial velocities, and astrometric programs on the ESO-MPIA 2.2m and
Palomar 1.5m telescopes to obtain transverse velocities. These data will
be used to address the question on everyone's lips - are the lowest mass disk stars
members of a young (~< 10^{8} yr) population, and therefore brown dwarfs?
Orbital Evolution of Extra-Solar Giant Planets
David E. Trilling (1), Willy Benz (1,2), Tristan
Guillot (3), and Jonathan I. Lunine (1)
The recent discoveries (Mayor and Queloz, 1995; Marcy and Butler, 1996;
Butler and Marcy, 1996) of extra-solar giant planets (EGPs) at small
heliocentric distances have prompted questions about the formation,
evolution, and migration of these EGPs.
The location of several EGPs at much less than 1 AU from
their primaries has proved to be particularly problematic.
Since it is thought that EGPs do not form that close to their primaries
(Guillot et al., 1996), a reasonable conclusion is that these close companions
formed elsewhere in their solar systems and subsequently moved to their present
small heliocentric distances. Jupiter-style planetary formation is thought
to initiate at the ice line (~several AU) (Boss, 1995), with possible
subsequent inward migration (Lin and Papaloizou, 1986). But how can a
massive body stop its inward migration before crashing into its star?
We investigate the orbital evolution of EGPs with our fully implicit numerical
model. Inward migration is caused by angular momentum exchange between the
planet and disk (Takeuchi et al., 1996; Lin and
Papaloizou, 1986). After the planet has migrated to much less than 1 AU,
its inward motion is halted by outward torques due to tides
between the star and the planet
(Lin et al., 1996) and due to angular momentum
exchange from Roche lobe overflow and mass loss
(Benz et al., 1990). We use state of the art
equation of state calculations
(Saumon et al., 1996; Guillot et al., 1996) to determine both the
interior structure and the planetary radius of
the migrating companion, as both
play critical roles in
the magnitude of the Roche lobe overflow and
therefore of the outward torque due to mass
loss. Migrating companions halt due to these
outward torques, and do not fall into their
stars (Trilling et al., 1996).
We investigate a wide range
of disk and planetary parameters to
determine the
ultimate fates of low
mass companions migrating inward
under various conditions.
This mechanism of halting inward
migration may explain the presence
of EGPs at small heliocentric distances.
References:
Acknowledgement: This work is supported in part under an
NSF Graduate Research Fellowship.
The Effective Temperature and Metallicity of CM Draconis
Serena Viti (1), Hugh R. A. Jones (2,3), Andreas Schweitzer (4,5),
France Allard (6), Peter H. Hauschildt (7), Jonathan Tennyson (1)
We compare observations of the
binary system CM Draconis with synthetic spectra computed using the stellar
atmosphere code PHOENIX. Spectroscopic
observations from 0.40 to 2.41 micron, combined with photometry and
the accurately known surface gravity enable us to derive the temperature
and metallicity using detailed spectra synthesis as well as the spectral
energy distribution. This enables us to obtain
the most direct measurement of both metallicity [M/H] and effective
temperature (Teff) so far made for the system.
We found discrepancies between the analysis of the infrared and optical
spectrum.
A Comparison of the Infrared Spectra of Two Very
Late-type M Dwarfs with Different Gravities
Serena Viti (1), Hugh R. A. Jones (2,3), Jonathan Tennyson (1),
France Allard (4), Peter H. Hauschildt (5)
We present observations obtained during the commissioning of the
upgraded Cooled Grating Spectrometer 4 on the UK Infrared Telescope on
Mauna Kea, of the very low-mass objects TVLM 513-46546 and
Gl 569B. Spectra were taken from 1.0 to 2.5 micron allowing the
peak of their energy distributions to be measured. TVLM 513-46546 and
Gl 569B have very similar spectral types and colours but they differ
by more than a magnitude in luminosity; this indicates that their
surface gravities differ by around 0.5 dex.
We interpret their stellar parameters by making comparisons
with the latest atmospheric models.
Nomenclature for Extrasolar Substellar Objects: A New Challenge
Wayne H. Warren Jr., and Helene R. Dickel
The recent discoveries of extrasolar substellar objects pose additional
complications for a nomenclature system (for multiple stars) that is already
not entirely consistent. The current situation will probably become even
more serious with the inevitable eventual discovery of planetary satellites
outside the Solar System. This paper reviews the current system for double
and multiple stars, discusses a number of suggestions already made for
designating substellar objects, and proposes additional possibilities for
an internally consistent and yet straightforward system.
Characterization of Extra-solar Planets via Doppler-modulation of Their IR Spectra
G. Wiedemann
The orbital motion of an extra-solar planet causes a periodic Doppler-shift
of its own spectrum relative to its host star. This global frequency modulation
may be exploited to extract a small planetary signature from an observed
stellar signal. The search for the infrared spectra of gaseous planet has been
simulated. A search strategy with a large telescope will be discussed.
About extra-solar comets in young stellar systems
D. de Winter (1), C.A. Grady (2), and C. Eiroa (1)
Planetary systems, such as our own solar system or those identified by reflex
motion studies, are thought to originate from disk-like structures around young
stars. In order to establish that the latter go into a transition phase of
planetesimals it is needed to search for evidence of conglomerated material.
Inhomogeneous structures as seen in the beta Pic disk may provide an
example of objects in such a transition phase. Spectroscopic studies of
this object have documented the presence of redshifted absorption lines
with velocities up to 300-400 km/s.
This ``beta Pic phenomenon''
is not limited to this nearby field A star, but is observed in other
field A-shell stars and more importantly, in the line of sight to a
number of beta Pic precursors, the Herbig Ae/Be stars. In addition
to the spectroscopic variability many of these objects also exhibit
anti-correlated photometric and polarimetric variability which has been
interpreted as occultation of the star by large dust clouds in a circumstellar
disk.
Due to the large accretion velocities, these infalling features provide
us with a probe of the circumstellar gas in the immediate vicinity of
the star, and thus sample material in the equivalent of a terrestrial
planet region. Over the past few years a number of multi-wavelength
studies of these objects have been carried out and the results of these
studies will be reviewed. In particular,
combined optical and UV spectroscopic studies of these stars have
demonstrated that accreting gas features with velocities, ionization
stages, and line profiles similar to those seen toward beta Pic
are routinely detected. Monitoring studies have shown that high velocity
features are more commonly detected in the line of sight to the
Herbig Ae/Be stars than toward beta Pic, that high accretion rate
episodes in these younger objects last up to several days
and that their accreting gas composition is more volatile rich than seen
towards beta Pic and main sequence A-shell stars. In some cases,
refractory elements show significant departures from
``cosmic'' abundances, consistent with the accreting gas having been
processed through the solid phase.
Together with the large infall
velocities, which exclude accretion of small grains in the intense radiation
fields of intermediate-mass PMS stars, the data are consistent with
detection of the gaseous comae of star-grazing bodies resembling our
expectations for planetesimals.
The presentation will be an up-to-date picture of the evidences of cometesimals
in the line of sight of Herbig Ae/Be and of near ZAMS A-shell type stars.
It will cover optical spectroscopic monitoring data, IUE and possibly ISO
spectra, which informs about velocity, ionization, covering factors,
composition, and time scale of variability of the accreting gas.
Planet Finder Options IV:
Mechanical aspects of a planet finder mission
N.J. Woolf
The extremely tight optical tolerance on phase
control, individual telescope and system pointing
and the relative positioning of the telescopes for
a planet finder mission pose problems for either
a mechanical structure to hold telescopes in space, or
for the control and movement of a set of free-flying
telescopes. There are also problems in the pointing
change from star to star. The choice of planet finder
mission form also affects the choice of appropriate
mission(s) to precede a planet finder mission. The
paper will list and discuss the problems that need to
be overcome to use either concept.
Formation of Giant Planets: Close to Stars and Further Out
G. Wuchterl
The formation of giant planets is studied by hydrodynamical
calculations of the gas-accretion caused by the gravitational
pull of growing terrestrial planets at distances of 0.05 to 5 au
from a solar mass star. The equations of radiation hydrodynamics in
spherical symmetry are solved for the gas flow from the nebula onto
the growing condensible element core. Conditions for the onset of
rapid gas accretion are discussed and nonlinear calculations of the
accumulation of a Jupiter mass are presented.
For a minimum mass solar nebula a giant planet
can form at 0.05 au if a solid core of 7 earth
masses is accumulated.
Caustic Singularities of the Cusp Type in the
Theory of Gravitational Lenses
A.F. Zakharov
The common solution of the gravitational lens equation near cusps was
presented. We used the gravitational lens equation by Schneider \&
Weiss. Using the symmetrical polynomials oh the roots of polynomial of third
degree we obtained the weak generalization of Schneider \& Weiss
statement on the magnification near different solutions of the gravitational
lens equation.
The analytical expressions for magnifications of different images near cusp
were presented. It is well known when we consider mappings of two dimensional
surface into plane that there are only two types of
stable singularities: folds and cusps (pleats).
There are also similar singularities of caustics in gravitational lens optics.
Schneider & Weiss 1986; Schneider & Weiss 1992
studied gravitational lens mapping near cusps. Some properties of
the mappings are very important for solving of different problems
of gravitational lensing, for example, in consideration of the
mutual coherence of images near the cusps (Mandzhos 1993).
The analitical expressions are very useful for these purposes.
Therefore, we will obtain the analytical expressions for solution
of gravitational lens equation and magnifications of different images
near the cusp.
Microlensing by non-Compact Astronomical Objects
A.F. Zakharov (1) and M. V. Sazhin (2)
We consider microlensing by neutralino stars.
The neutralino stars were analyzed recently
by Gurevich and Zybin, and besides
the stars are considerable component of dark matter.
We consider the optics of the microlens, namely
lens equation, the magnification, critical and caustic curves.
We discuss also criteria to distinguish
non-compact and compact microlenses.
We consider the distortion of the gravitational microlens
model by gravitational field of our Galaxy.
We use the Chang - Refsdal lens for our analysis.
The detailed discussions of the clear model is presented.
We show that the influence of gravitational field of our Galaxy
during microlensing is not very large in greater part of observations.
The Distortion of Microlensing by Mass Distribution of our Galaxy
A.F. Zakharov (1) and M. V. Sazhin (2)
We consider the distortion of standard gravitational microlens
model by gravitational field of our Galaxy.
We use the Chang - Refsdal lens for our analysis.
The detailed discussions of the clear model is presented.
We show that the influence of gravitational field of our Galaxy
during microlensing is not very large in greater part of observations.
We discuss also a possibility to interpretate event OGLE No. 7, using the
model.
Revealing the Brown Dwarf Population in the Pleiades Cluster
M. R. Zapatero Osorio
We present results of an observational project carried out with the telescopes
on the Canary observatories, and aimed at revealing the brown dwarf population
in the Pleiades young open cluster. We have conducted deep CCD R, I and Z mosaic
imaging of a area of ~1 square degree (6% of the total area of the
cluster), reaching limiting magnitudes of R=23.0, I=22.0 and Z=21.0 (the
completeness of the survey is about 1 mag brighter). According to recent
theoretical evolutionary models and assuming a cluster age of 100 Myr, we
are sensitive to objects as low in mass as 30 M_J. As a result of our
photometric search, a relatively large number of substellar candidates has
arisen with I magnitudes in the range ~18-20. Follow-up infrared and narrow-band
filter photometry of most of our objects indicate that they are consistent
with the Pleiades substellar sequence defined by the lithium-confirmed brown
dwarfs. For a few candidates we have also obtained low- and mid-resolution
optical spectroscopy confirming that they are indeed very late-M type dwarfs,
and that they have radial velocities which are consistent with cluster
membership. The faintest objects in our sample appear to define an extension
of the Pleiades substellar sequence beyond Teide 1 and Calar 3.
Search for Brown Dwarfs with ISO
H. Zinnecker, H. Jahreiss, C. Leinert, M.J. McCaughrean
In an ongoing project, we are using ISOCAM to search
for brown dwarfs as companions to some 40 nearby
white dwarfs within 15 pc from the Sun selected from the
Gliese/Jahreiss Catalog. These white dwarfs are old
objects (several Gyr) so that any companion brown dwarfs
are expected to have cooled down to 200-400 K, ideal for
detection of an infrared excess in the 7-15 micron range.
In fact, white dwarfs (rather than red M dwarfs) are the
perfect targets to search for brown dwarf companions, as
their radius is 10 times smaller than brown dwarf radii
so that the brown dwarf blackbody emission curve can
cross and exceed the white dwarf blackbody emission in
the infrared.
Our search is different from the Zuckerman & Becklin (1987,
Nature 330, 138) and Barnbaum & Zuckerman (1992, ApJ 396, L31)
surveys which concentrated on YOUNG (< 1 Gyr) white dwarfs
so that any young brown dwarf companions can be detected
from the ground at 2 micron. By choosing a sample of OLD
white dwarfs we make sure that any infrared companion must
be a bona fide brown dwarf, i.e. an object with a
temperature much lower than that of an M8-M10 Main Sequence
star, hence a mass below the hydrogen burning limit.
We need ISOCAM, because ground-based mid-infrared
observations do not have the necessary sensitivity by a
factor of 10-100 (of order 1 mJy at 10 micron is needed).
ISOCAM (3"/pixel) allows us to register any infrared excess
emission coincident with the white dwarf and to ensure that there is
no confusion with cool background objects, e.g. obscured
high-z starburst galaxies which we may happen to detect
(cf. the recent ISOCAM data on the Hubble Deep Field).
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(1) European Southern Observatory
(2) European Space Agency
Department of Physics and Astronomy, Leicester University, University Road,
Leicester LE1 7RH, U.K.
(1) Paris-Meudon Observatory, France
(2) LMD/CNRS, Univ. of Paris 6, France
(3) Inst. for Astronomy, Hawaii, USA
(4) Rutherford-Appleton Lab., Oxon, UK
(5) Reading Meteorology Lab, UK
Instituto de Astrofísica de Canarias. E-38200 La Laguna, Tenerife, Spain.
(1) Observatoire de Grenoble, 414 rue de la Piscine,
Domaine Universitaire de St. Martin d'Hères,
F-38041 Grenoble,
France
(2) Observatoire de Genève
CH-1290 Sauverny,
Switzerland
SETI Institute, NASA Ames Research Center. MS 245-3, 94035 Moffett Field, California, USA
(1) Observatoire de Bordeaux, B.P. 89, F-33270 Floirac, France
(2) Sternwarte der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
Astronomical Observatory in Uppsala, S-751 20 UPPSALA, Sweden
(1) Observatoire de Grenoble, 414 rue de la Piscine, Domaine Universitaire,
BP 53, F38041 Grenoble Cedex, France
(2) Anglo Australian Observatory, PO Box 296 Epping NSW 2121 (167 Vimiera Road, Eastwood, NSW 2122), Australia
Palomar Observatory, Caltech, Mailstop 105-24, Pasadena CA 91125, USA
California Institute of Technology, Mailstop 105-24, Pasadena CA 91125, USA
University of Washington, Astronomy Department, Box
351580, Seattle, WA 98195-1580, USA
(1) Dept. of Physics and Astronomy, Northern Arizona University, Flagstaff,
AZ, 86011, USA
(2) New Mexico State Univ., Astronomy Department, Las Cruces, NM,
88003
(3) Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
(1) Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, CA 91109, USA
(2) Departamento de Astronomía, Universitat de València, 46100
Burjassot, Valencia, Spain
(3) Australian Telescope National Facility, Epping, New South Wales 2121,
Australia
(4) Observatoire de Paris-Meudon-CNRS, F-92195 Meudon Principal Cedex, France
(5) University of Tasmania, Hobart, Tasmania 7001, Australia
(6) U.S. Naval Observatory, Washington D.C., 20392, USA
(7) University of Western Sydney, Sydney, New South Wales, Australia
(8) Observatoire de la Côte d'Azur, CERGA, F-06130 Grasse, France
(9) Lund Observatory, S-22100 Lund, Sweeden
(1) Observatoire de Strasbourg, 11 rue de l'université, F-67000
Strasbourg, France
(2) Observatoire de Genève, 51 chemin des Maillettes, CH-1290 Sauverny,
Switzerland
(Invited review)
Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, Scotland, UK
McDonald Observatory, Univ. of Texas at Austin, Austin,
TX 78712, USA
(Invited review)
McDonald Observatory, Univ. of Texas at Austin, Austin,
TX 78712, USA
(1) Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
(2) Bordeaux Observatory, Bordeaux, France
(3) Astronomy Group, LMU, Liverpool L3 3AF, UK
(4) Institute of Astronomy, University of Tokyo, Mitaka 181, Japan
(Invited review)
(1) Institute of Astronomy, University of Tokyo
(2)Astrophysics Group, Liverpool John Moores University
(1) Institute of Astronomy, University of Tokyo
(2) Astrophysics Group, Liverpool John Moores University
(1) Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden
(2) Uppsala Astronomical Observatory, Box 515, S-751 20 Uppsala, Sweden
Max-Planck Institute for Astronomy, Konigstuhl 17, D-69117 Heidelberg
(Invited review)
University of California at Los Angeles, Dept. of Physics
& Astronomy, P. O. Box 951562, Los Angeles, CA 90095-1562, USA
(Invited review)
Astronomisches Rechen-Institut, Mönchhofstr. 12-14,
D-69120 Heidelberg, Germany
Institute of Astronomy of the Russian Acad. Sci., 48 Pyatnitskaya str., Moscow 109017, Russia
(1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA 02138 USA
(2) School of Physics and Astronomy, Raymond and Beverly Sackler
Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel
(3) Department of Physics and Astronomy, University of North Carolina,
Chapel Hill, NC 27599-3255 USA
(1) Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany
(2) Department of Physics, Wichita State University, 1845 Fairmount, KS 67260-0032 Wichita, USA
(3) Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy
Steward Observatory, University of Arizona, Tucson, AZ 85721
Osservatorio Astrofisico di Catania, Italy
European Southern Observatory, Karl-Schwarzschild Strasse, 2,
D-85748 Garching, Germany
(Invited review)
(1) San Francisco State University, Dept. of Physics, SFSU, SF, CA 94132, USA
(2) University of California, Berkeley, Astronomy Dept., Berkeley, CA 94720, USA
(Invited review)
NMSU; P.O. Box 30001; Las Cruces NM 88003
Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
(Invited review)
Observatoire de Genève, 51 chemin des Maillettes,
CH-1290 Sauverny, Switzerland
Max-Planck-Institute für Astronomie, Heidelberg, Germany
(1) School of Physics and Astronomy, Raymond and Beverly Sackler
Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel
(2) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA 02138 USA
Department of Physics and Astronomy, Denison University, Granville, Ohio 43023 USA
(Invited review)
Palomar Observatory, Caltech, MS 105-24, Pasadena, CA 91125, USA
Lawrence Livermore National Laboratory
Stockholm Observatory, S-13336 Saltsjobaden, Sweden
(1) California Institute of Technology, 105-24, Pasadena, CA 91125
(2) Johns Hopkins Univeristy, Dept. of Physics and Astronomy, Baltimore,
MD 21218
(1) California Institute of Technology, Pasadena, CA 91125
(2) Jet Propulsion Laboratory, Pasadena, CA 91109
(Invited review)
European Southern Observatory, Karl Schwarzschild Str. 2,
85748 Garching b. Munchen, Germany
The Main Observatory of Academy of Sciences of Ukraine,
Golosiiv wood, 252650 Kyiv-22, UKRAINE
1.- An overall shape of the observed spectra may be reasonably well fitted by
computations in the visible region, including spectral details around the
strongest Li Lines.
2.- The strength of molecular bands formed the background for Li I lines
depends on Teff and metallicity mainly. For Teide 1 and Calar 3
2700 < Teff < 3000 K.
3.- The fit of moderate resolution spectra of Teide 1 and Calar 3 showed
that lithium abundances in their atmospheres are rather high:
log N(Li) > 2.5.
(1) Rutherford Appleton Lab., Didcot, UK;
(2) U. A. de Madrid, Madrid, Spain;
(3) ESTEC, Noordwijk, Netherlands;
(4) SRON, Groningen, Netherlands;
(5) IAS, Orsay, France;
(6) Obs. Meudon, Paris, France;
(7) DLR, Berlin, Germany
Dept. of Physics and Astronomy, University of Leicester, Leicester, United Kingdom
Geneva Observatory (Switzerland)
Canada France Hawaii Telescope (USA)
Grenoble Observatory (France)
Haute Provence Observatory (France)
UCLA Astronomy & Astrophysics, Box 951562, Los Angeles, CA 90095-1562, USA
(1) Landessternwarte Heidelberg, Königstuhl, D-69117 Heidelberg, Germany
(2) Department of Physics and Astronomy, The Universuty of Georgia, Athens, GA 30602-2451
(3) Department of Physics, The Wichita State University, Wichita, KS 67260-0032
Harvard-Smithsonian Center for Astrophysics, Mail Stop 66, 60 Garden St.,
Cambridge, MA 02138, USA
(1) Liverpool John Moores University, UK
(2) Royal Observatory, Blackford Hill, Edinburgh, EH9~3HJ, Scotland, UK
(3) Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
(4) Department of Physics and Astronomy, Leicester University,
University Road, Leicester LE1 7RH, UK
National Astronomical Observatory, Osawa 2-21-1, Mitaka, Tokyo 181,
Japan (tamuramt@cc.nao.ac.jp)
(1) Anglo-Australian Observatory. PO Box 296. Epping. 2121. Australia
(2) Palomar Observatory
(3) Mount Stromlo & Siding Spring Observatory
(1) Department of Planetary Sciences and
Lunar and Planetary Lab, Univ. of Arizona, Tucson,
AZ 85721, USA
(2) Department of Astronomy, Univ. of Arizona,
Tucson, AZ 85721, USA
(3) Department of Meteorology, Univ. of Reading,
PO Box 239, Whiteknights, Reading RG6 6AU, UK
Benz et al., 1990, Ap. J., 348, 647.
Boss, 1995, Science, 267, 360.
Butler and Marcy, 1996, Ap. J., 464, L153.
Guillot et al., 1996, Ap. J., 459, L35.
Lin et al., 1996, Nature, 380, 606.
Lin and Papaloizou, 1986, Ap. J., 309, 846.
Marcy and Butler, 1996, Ap. J., 464, L147.
Mayor and Queloz, 1995, Nature, 378, 355.
Saumon et al., 1996, Ap. J., f460, 993.
Takeuchi et al., 1996, Ap. J., 460, 832.
Trilling et al., 1996 B. A. A. S., 28, 1113.
Zuckerman et al., 1995, Nature, 373, 494.
(1) University College London
(2) University of Tokyo
(3) LJMU
(4) Landessternwarte Heidelberg
(5) ASU
(6) Wichita State University
(7) University of Georgia
(1) University College London
(2) University of Tokyo
(3) LJMU
(4) Wichita State University
(5) University of Georgia
HSTX NASA Goddard SFC, Greenbelt, Maryland 20771-1000, USA
European Southern Observatory, 85748 Garching, Germany
(1) Dpto. Física Teórica, C-XI, Facultad de Ciencias, Universidad
Autónoma Madrid, Cantoblanco, E-28049 Madrid, Spain
(2) Eureka Scientific, 2452 Delmar St., Suite 100, Oakland, CA 94602, USA
Center for Astronomical Adaptive Optics,
Steward Observatory,
University of Arizona,
Tucson AZ 85721, USA
Inst. f. Astronomie der Univ. Wien, Tuerkenschanzstrasse 17 ,
A-1180 Wien, Austria
Institute of Theoretical and Experimental Physics, B.
Cheremushkinskaya, 25, 117259, Moscow, Russia
(1) Institute of Theoretical and Experimental Physics, B. Cheremushkinskaya,
25, 117259, Moscow, Russia
(2) Sternberg State Astronomical Institute, Universitetsky Pr. 13, 117234,
Moscow, Russia
(1) Institute of Theoretical and Experimental Physics, B. Cheremushkinskaya,
25, 117259, Moscow, Russia
(2) Sternberg State Astronomical Institute, Universitetsky Pr. 13, 117234,
Moscow, Russia
Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife. Spain
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