A&A 442, 961-992 (2005)
The lithium content of the Galactic Halo starsC. Charbonnel1, 2 and F. Primas3, 2 1
Geneva Observatory, 51 chemin des Maillettes, 1290 Sauverny,
Laboratoire d'Astrophysique de Toulouse et de Tarbes, CNRS UMR 5572, 14 Av. E.
Belin, 31400 Toulouse, France
European Southern Observatory, Karl-Schwarzschild Str. 2,
85748 Garching b. München, Germany
(Received 7 December 2004 / Accepted 8 May 2005 )
Thanks to the accurate determination of the baryon density of the universe
by the recent cosmic microwave background experiments, updated predictions
of the standard model of Big Bang nucleosynthesis now yield the initial
abundance of the primordial light elements with unprecedented precision.
In the case of 7Li, the CMB+SBBN value is significantly higher than the
generally reported abundances for Pop II stars along the so-called Spite
plateau. In view of the crucial importance of this disagreement, which has
cosmological, galactic and stellar implications, we decided to tackle the
most critical issues of the problem by revisiting a large sample of
literature Li data in halo stars that we assembled following some strict
selection criteria on the quality of the original analyses.
In the first part of the paper we focus on the systematic uncertainties
affecting the determination of the Li abundances, one of our main goal
being to look for the "highest observational accuracy achievable" for
one of the largest sets of Li abundances ever assembled. We explore in
great detail the temperature scale issue with a special emphasis on reddening.
We derive four sets of effective temperatures by applying the same
calibration but making four different assumptions about
reddening and determine the LTE lithium values for each of them. We compute
the NLTE corrections and apply them to the LTE lithium abundances.
We then focus on our "best" (i.e. most consistent) set of temperatures
in order to discuss the inferred mean Li value and dispersion in several
and metallicity intervals. The resulting mean Li values along the
plateau for [Fe/H]
when the lowest
effective temperature considered is taken equal to 5700 K and 6000 K
respectively. This is a factor of ~2.48 to 2.81
(depending on the adopted SBBN model and on the
effective temperature range chosen to delimit the plateau)
lower than the CMB+SBBN determination. We find no evidence of intrinsic
Assuming the correctness of the CMB+SBBN prediction, we are
then left with the conclusion that the Li abundance along the plateau is
not the pristine one, but that halo stars have undergone surface depletion
during their evolution.
In the second part of the paper we further dissect our sample in search
of new constraints on Li depletion in halo stars. By means of the Hipparcos
parallaxes, we derive the evolutionary status of each of our sample stars,
and re-discuss our derived Li abundances. A very surprising result emerges
for the first time from this examination.
Namely, the mean Li value as well as the dispersion
appear to be lower (although fully compatible within the errors) for the
dwarfs than for the turnoff and subgiant stars.
For our most homogeneous dwarfs-only sample with [Fe/H]
the mean Li abundances are
when the lowest
effective temperature considered is taken equal to 5700 K
and 6000 K respectively. This is a factor of 2.52 to 3.06
(depending on the selected range in
for the plateau and on the SBBN
predictions we compare to) lower than the CMB+SBBN primordial value.
Instead, for the post-main sequence stars the corresponding
, which correspond to a
depletion factor of 2.28 to 2.52.
These results, together with the finding that all the stars with Li
abnormalities (strong deficiency or high content) lie on or originate
from the hot side of the plateau, lead us to suggest that the most massive
of the halo stars have had a slightly different Li history than their less
massive contemporaries. In turn, this puts strong new constraints on the
possible depletion mechanisms and reinforces Li as
a stellar tomographer.
stars: abundances --
stars: Population II --
stars: evolution --
Galaxy: halo --
cosmology: early Universe
© ESO 2005