Issue |
A&A
Volume 413, Number 3, January III 2004
|
|
---|---|---|
Page(s) | 1045 - 1063 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20031582 | |
Published online | 07 January 2004 |
Abundances of Na, Mg and Al in nearby metal-poor stars*
1
Institut für Astronomie und Astrophysik der Universität München, Scheinerstr. 1, 81679 München, Germany
2
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
3
Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China
Corresponding author: T. Gehren, gehren@usm.uni-muenchen.de
Received:
17
June
2003
Accepted:
7
October
2003
To determine the population membership of nearby stars we explore
abundance results obtained for the light neutron-rich elements 23Na and Al in
a small sample of moderately metal-poor stars. Spectroscopic observations are
limited to the solar neighbourhood so that gravities can be determined from
Hipparcos parallaxes, and the results are confronted with those for a separate
sample of more metal-poor typical halo stars. Following earlier investigations,
the abundances of Na, Mg and Al have been derived from NLTE statistical
equilibrium calculations used as input to line profile synthesis. Compared with
LTE the abundances require systematic corrections, with typical values of
+0.05 for [Mg/Fe], -0.1 for [Na/Fe] and +0.2 for [Al/Fe] in thick disk
stars where [Fe/H]
. In more metal-poor halo stars these values reach
+0.1, -0.4, and +0.5, respectively, differences that can no longer be
ignored.
After careful selection of a clean subsample free from suspected or known
binaries and peculiar stars, we find that [Na/Mg] and [Al/Mg], in combination
with [Mg/Fe], space velocities and stellar evolutionary ages, make possible an
individual discrimination between thick disk and halo stars. At present, this
evidence is limited by the small number of stars analyzed. We identify a gap at
[Al/Mg]
and [Fe/H]
that isolates stars of the thick
disk from those in the halo. A similar separation occurs at [Na/Mg]
.
We do not confirm the age gap between thin and thick disk found by Fuhrmann.
Instead we find an age boundary between halo and thick disk stars, however, with
an absolute value of 14 Gyr that must be considered as preliminary.
While the stellar sample is by no means complete, the resulting abundances
indicate the necessity to revise current models of chemical evolution and/or
stellar nucleosynthesis to allow for an adequate production of neutron-rich
species in early stellar generations.
Key words: line: formation / line: profiles / stars: fundamental parameters / stars: abundances / stars: late-type
© ESO, 2004
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