Issue |
A&A
Volume 375, Number 1, August III 2001
|
|
---|---|---|
Page(s) | 70 - 86 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20010848 | |
Published online | 15 August 2001 |
On the possible existence of a self-regulating hydrodynamical process in slowly rotating stars
II. Lithium plateau in halo stars and primordial abundance
Laboratoire d'Astrophysique, 14 Av. E. Belin, 31400 Toulouse, France
Corresponding author: S. Théado, stheado@obs-mip.fr
Received:
26
December
2000
Accepted:
25
May
2001
The lithium plateau observed in halo stars has long appeared as a
paradox in the general context of the lithium abundance behavior in stellar outer
layers. First, the plateau is flat, second, the lithium abundance dispersion is
extremely small. This seems in contradiction with the large lithium variations observed
in younger stars. It is also difficult to understand theoretically: as lithium
nuclei are destroyed by nuclear reactions at a relatively low temperature
(≅2.5 million degrees), the occurrence of macroscopic motions in the
stellar outer layers easily lead to lithium depletion at the surface. On the
other hand, if no macroscopic motions occur in the stellar gas, lithium is subject
to microscopic diffusion which, in the case of halo stars, should also lead
to depletion. Several ideas have been proposed to account for the lithium behavior in
halo stars. The most promising possibilities were rotational-induced mixing, which
could reduce lithium in the same way for all the stars (Vauclair [CITE]; Pinsonneault
et al. [CITE] and [CITE]) and mass-loss, which could oppose the
lithium settling (Vauclair & Charbonnel [CITE], [CITE]). In both cases however, the parameters should be tightly adjusted
to prevent any dispersion in the final results.
Vauclair ([CITE]) (Paper I) looked for a physical process
which could occur in slowly rotating stars and explain why the dispersion of the lithium
abundances in the halo stars' plateau is so small. She pointed out
that the -gradient terms which appear in the
computations of the meridional circulation velocity
(e.g. Mestel [CITE]) were
not introduced in previous computations of rotationally-induced mixing. This can lead
to a self-regulating process which reduces the efficiency of the
meridional circulation as well as the microscopic diffusion.
Here we present numerical computations of this process and its influence
on the lithium abundance variations in halo stars.
We show that in slowly rotating stars, under some conditions, lithium can be depleted
by a factor of up to two with a dispersion smaller than 0.1 dex in the
middle part of the lithium plateau. We derive a primordial
lithium abundance of
, consistent with the recent determinations of
D/H and 4He/H.
Key words: diffusion / hydrodynamics / stars: abundances / stars: population II
© ESO, 2001
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.