The lithium observed in old stars has a uniform abundance: is this the lithium
of the Big-Bang?
Lithium is one of the elements (with deuterium) formed primarily during the
primordial nucleosynthesis, a few minutes after the
Big-Bang. Its abundance thus allows to determine the quantity of baryons in the Universe. It proves that the density of
baryons deduced (or its value
normalized to the
critical density
to close the Universe) is much larger than the one
visible by radiation. It is therefore essential to better constrain this density of baryons. A recent work on the ESO-VLT
of an international consortium, in which take part several astronomers of the Paris Observatory, brings
confirmation of the constancy of the lithium abundance in stars, which suggests that the abundance observed is quite close
to the primordial abundance, and thus makes it possible to refine the determination of
.
The problem
The problem of the abundance of lithium in old stars has fed a discussion for years: is the
observed abundance that of the Big-Bang, preserved
intact? or was it decreased by various stellar phenomena (stellar winds, convective and/or rotational mixing, diffusion, destruction in deep layers)
and possibly increased by production through cosmic rays? The quantity of lithium currently observed in the external layers of the Sun is
definitely weaker than the initial quantity (known thanks to the meteorites), and solar helium shows the existence of diffusion phenomena.
But old stars are low in metals, they have a structure (and a history) different from that of the Sun. Several theories proposed in the past that
the initial abundance of lithium (that of the Big-Bang) in these old stars was depleted by 90% in a Hubble time. It would then have been
necessary to multiply by 10 the observed abundance to obtain the primordial one. However the calculated depletion varies according to
the effective temperature, to the mass, the metallicity, the stellar rotation, so that one should then observe a strong scatter of the remaining
lithium abundances, but this scatter is not observed.
The current theories predict a weaker reduction, and consequently a weaker scatter. Measurements are not perfect, and give a weak
scatter due to errors. It becomes difficult to disentangle the measurement errors
from the variations due to the possible reduction processes.
Figure 1.
Observation of the lithium line in 5 stars of the globular cluster NGC 6397. The
measurement error is indicated in each point of the absorption line. The line is
dissymmetrical, because it is produced by a doublet with unequal components.
Three synthetic profiles are calculated, the best fit is the medium one.
At each curve corresponds an equivalent width (EW). The equivalent widths
are different from one star to another, because the temperatures are different,
but lithium abundances are very close.
Click on the figure to enlarge it.
A recent progress
An international team, directed by R. Gratton, and including three astrophysicists at
Paris Observatory
(F. Spite, P. François et G. James)
undertook with the VLT a study of old and metal-poor stars: globular cluster and field
stars, in order in particular to
evaluate the amplitude of the mixing and/or diffusion effects in these stars. Several elements (of which lithium) were measured in a
homogeneous way. With regard to the other elements, it appears that the diffusion
effect is less than considered in certain theoretical
calculations. With regard to lithium, P. Bonifacio
and his collaborators undertook a thorough study of measurement errors on a dozen stars of
the globular cluster
NGC 6397 and field stars. The reduction software was carefully checked. In the transformation of line measurements (equivalent widths) into
abundances, the errors were reduced by the use of an original method, avoiding photometry
errors and the uncertainty in interstellar
reddenings. To increase the statistics, archive VLT spectra were used in complement: 12 stars thus could be observed with several
spectra for each star. An analysis of the observed scatter and a Monte Carlo simulation show that measurement errors, although very weak,
explain alone almost entirely the observed scatter (0.0622): the intrinsic dispersion of lithium abundances in stars is very weak (0.0352), and
consequently the possible depletions in lithium can only be very weak.
The field stars, measured in a similar way give the same lithium abundance.
Thus, lithium abundances of old stars provide a very good estimation of the
primordial nucleosynthesis lithium abundance: NLi/NH = 2.1
10-10. And the predictions of this nucleosynthesis imply a ratio between the number of baryons and the number of photons Nb/Np = 4.3 10-10,
that is to say a normalized baryon density
h2
= 0.016 + 0.004 (where h is the
Hubble constant, normalised to 100km/s/Mpc),
in satisfactory agreement with the determinations of primordial
abundances of D, 3He, He and with the anisotropies of the
cosmological background within the uncertainties.
The problem of lithium is not entirely solved. The sample should be extended, which
will be soon possible to do economically
by using the
multi-objects spectroscopy on VLT (FLAMES). The nitrogen abundance should be better understood. The contribution of
cosmic rays should be better estimated. The cause of some rare aberrant lithium abundances should be cleared up, as well as the effect of
binaries.
Reference:
P. Bonifacio, L. Pasquini, F. Spite, A. Bragaglia, E. Carretta, V. Castellani,
M. Centurion, A. Chieffi, R. Claudi, G. Clementini, F. D'Antona, S. Desidera, P.
Francois, R.G. Gratton, F. Grundahl, G. James, S. Lucatello, C. Sneden), O.
Straniero
"The lithium content of the globular cluster NGC 6397"
Astronomy and Astrophysics, accepted
astro-ph/0204332
Contact:
François Spite (Observatoire de Paris, GEPI)
Patrick François
(Observatoire de Paris, GEPI)
Gaël James (Observatoire de Paris, GEPI)
