On October 29, 2001, the first fringes were obtained by the optical interferometer VLTI (ESO-VLT), by combining the light coming from the two 8m-telescopes ANTU (UT1) and MELIPAL (UT3) in the VINCI instrument, at Paranal. In the following days, interferometric measurements were done for several astronomical objects, including red dwarfs, supermassive variable stars, and Cepheids. Pierre Kervella from Paris-Meudon Observatory, and his collaborators, report about these exciting moments, and the importance of the Cepheid results.
The Very Large Telescope (VLT) of the
ESO Observatory
on Cerro Paranal (2635 m) in Northern Chile, is composed of four large telescopes,
that are meant to be used in an interferometric mode, the
VLTI.
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The VLT (Very Large Telescope) of ESO at Paranal (Chile) is composed of four telescopes of 8.2m diameter each, that are visible on this photograph at sunset. For the interferometric project, there are also auxiliary telescopes, of 1.8m in diameter. Click on the photo, to enlarge its size. |
| On March 17, 2001, the VLT Interferometer observed its first fringes on a star, using two siderostats and the test camera VINCI, built by the Paris-Meudon Observatory. Details on the experiment and about interferometry in general can be found in the references below. Click on the photo, to enlarge its size. |
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| First fringes obtained on Alpha Hydrae, in March 2001, between two small test telescopes of the VLTI. The diameter of the bright star has been determined to be 0.00928±0.00017 arcsec. |
Cepheids are variable supergiant stars: they undergo radial pulsations,
and their radius and luminosity oscillate with a period of the order of
5 to 60 days. They are known to obey a
tight relation P(L) between their absolute luminosity L and their oscillation
period P.
Once this relation is calibrated precisely for nearby Cepheids,
the absolute luminosity of remote Cepheids is derived from the measured period, and
therefore their
distance, and by the way also the distance of the objects (clusters,
galaxies) in which they are included.
Indeed, the distance of a star is easily derived from the comparison of its
absolute luminosity to its apparent luminosity. And thus a precise P(L) relation
converts the Cepheids into primary
distance indicators for extragalactic astronomy.
The calibration of the relation P(L) is delicate, however.
Even the nearest Cepheids are usually too far away for a direct measurement
of their distance (and absolute luminosity). The angular diameter is usually
not measurable from a single telescope.
In a first method, called the Baade-Wesselink method, the radial velocity
(velocimetry) and
the color (colorimetry) of the star is measured all along the period. Two
times (phases) are chosen in which the star has the same color but different
radii and different luminosities. The (common) temperature is evaluated from
the (common) color, the ratio of the radii is derived fom the ratio of the
apparent luminosities, the difference between the two radii is derived from
the integration of the velocity between the two selected times. A simple
algebra derives the two values of the radius, the absolute luminosity
follows from the knowledge of the radius and temperature (hence the
distance). The method is sensitive to the temperature error.
A better estimation is obtained, when the angular size of the star is
measured with an interferometer. Long baselines are required, since the
diameter is of the order of a few milli-arcsecond (mas).
The second method relies heavily on long baseline interferometers:
with the latter, the angular diameter of the Cepheid
variable star is measured at two different phases of its pulsation. This
provides a first number X: the amplitude of its angular size variation
between these two phases. This first result is usually expressed in
milliarcseconds.
Complementary observations by radial velocimetry (spectroscopic measurement
of the speed of the surface of the star) are then used to determine the
linear amplitude of the diameter variation of the Cepheid between the two
same phases as above. This second number Y is a linear value, expressed in
meters.
Using the two values X and Y measured above, it is possible to compute
directly the distance in parsecs to the star through the simple equation
d[pc] = 9.305 * Y[m] / X[mas].
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A second method to obtain the distances to the Cepheids Left: Measurement of the radial velocity (perpendicular to the sky plane) as a function of time Right: Direct measurement of the oscillations of the stellar radius |
Because of the turbulence of the atmosphere, which is worse at visible
wavelengths, interferometric measurements are easier in the near-infrared bands
(near 2 microns). The spatial resolution that can be obtained is proportional
to the distance between the interfering telescopes (the baseline).
Below are presented observations of the Cepheid Zeta Geminorum with the FLUOR beam combiner, installed at the IOTA interferometer. The mean uniform disk angular diameter was measured to be 1.64 +0.14 -0.16 mas. The distance to zeta Gem is evaluated by the first method above to be 502 +/- 88 pc.
FLUOR (Fiber Linked Unit for Optical Recombination) has served as a proto-type of the VINCI instrument, and is one of the three recombination instruments of IOTA (Infrared and Optical Telescope Array), a collaboration with 5 American Institutions, in Arizona.
The measurement was also done last November with the VLTI, with the interferences between
ANTU and MELIPAL (distant by a baseline of 102m).
The angular diameter was measured more accurately,
to be 1.78 +/- 0.02 mas.
Pulsational variations were not detected with FLUOR/IOTA, but are
expected to be measured with VINCI/VLTI.
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Left:
Study of the Cepheid Zeta Gem with FLUOR/IOTA in 1999/2000.
The pulsation is not detected, but the diameter is measured to be 1.64 +/- 0.16 mas Right: Model of the pulsation curve of Zeta Gem as will be observed with VINCI/VLTI, based on the precision achieved during the commissioning run (by P. Kervella) |
During 2002, the VLTI science instruments
MIDI
and
AMBER
and the fringe sensor unit
FINITO will arrive,
and the integration of the Auxiliary Telescopes will start. Once the ATs and the science
instruments are functional, regular science operations can start.
P. Kervella, V. Coudé du Foresto, G. Perrin, M.Schoeller, W. A. Traub, M. G. Lacasse, 2001, ``The angular diameter and distance of the Cepheid Zeta Geminorum'' Astronomy and Astrophysics, 367, 876 (astro-ph/0102359)
P. Kervella, Soutenance de thèse, le 14 Novembre 2001 power-point document
Giant Eyes for the VLT Interferometer First Scientific Results with Combined Light Beams from Two 8.2-m Unit Telescopes ESO Press Release 23/01, 5 November 2001
"First Light" for the VLT Interferometer - Excellent Fringes From Bright Stars Prove VLTI Concept ESO Press Release 06/01, 18 March 2001
Contact:
Pierre Kervella (DESPA, Observatoire de Meudon and ESO-Garching)
Vincent Coudé du Foresto (DESPA, Observatoire de Meudon),