Issue |
A&A
Volume 478, Number 2, February I 2008
|
|
---|---|---|
Page(s) | 529 - 541 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20078060 | |
Published online | 04 December 2007 |
Non-LTE line formation for heavy elements in four very metal-poor stars *
1
Institut für Astronomie und Astrophysik der Universität München, Scheinerstr. 1, 81679 München, Germany e-mail: lyuda@usm.lmu.de
2
Institute of Astronomy, Russian Academy of Science, Pyatnitskaya 48, 119017 Moscow, Russia e-mail: lima@inasan.ru
3
National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012, PR China
4
National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan
5
Liberal Arts Education Center, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan
6
Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China
Received:
11
June
2007
Accepted:
16
November
2007
Aims.Stellar parameters and abundances of Na,
Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very
metal-poor (VMP) stars (Fe/H
). For two
of them, HD 84937 and HD 122563, the fraction of the odd
isotopes of Ba derived for the first time.
Methods.Determination of an effective
temperature, surface gravity, and element abundances was based on
non-local thermodynamic equilibrium (non-LTE) line formation and
analysis of high-resolution ( and 90 000) high
signal-to-noise (
) observed spectra. A model atom for
is presented. An effective temperature was obtained
from the Balmer Hα and Hβ line wing fits. The
surface gravity was calculated from the Hipparcos parallax
if available and the non-LTE ionization balance between
and
. Based on the hyperfine structure
affecting the
resonance line
, the
fractional abundance of the odd isotopes of Ba was derived from a
requirement that Ba abundances from the resonance line and
subordinate lines of
must be equal.
Results.For each star, non-LTE leads to a
consistency of Teff from two Balmer lines and to a higher
temperature compared to the LTE case, by up to 60 K. Non-LTE
effects are important in spectroscopic determination of surface
gravity from the ionization balance between and
. For each star with a known trigonometric surface
gravity, non-LTE abundances from the lines of two ionization
stages,
and
, agree within the error bars,
while a difference in the LTE abundances consists of 0.23 dex to
0.40 dex for different stars. Departures from LTE are found to be
significant for all investigated atoms, and they strongly depend
on stellar parameters. For HD 84937, the Eu/Ba ratio is
consistent with the relative solar system r-process abundances,
and the fraction of the odd isotopes of Ba, fodd, equals
0.
. The latter can serve as an observational constraint
on r-process models. The lower Eu/Ba ratio and
found for HD 122563 suggest that the s-process or
the unknown process has contributed significantly to the Ba
abundance in this star.
Key words: line: formation / line: profiles / stars: abundances / stars: fundamental parameters / stars: late-type / stars: general
© ESO, 2008
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