Issue |
A&A
Volume 376, Number 1, September II 2001
|
|
---|---|---|
Page(s) | 232 - 247 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20010965 | |
Published online | 15 September 2001 |
Heavy element abundances in cool dwarf stars: An implication for the evolution of the Galaxy*
1
Department of Astronomy, Kazan State University, Kremlevskaya 18, Kazan 8, 420008, Russia
2
Institut für Astronomie und Astrophysik der Universität München, Scheinerstr. 1, 81679 München, Germany
3
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85740 Garching, Germany
Corresponding author: L. Mashonkina, Lyudmila.Mashonkina@ksu.ru
Received:
16
March
2001
Accepted:
4
July
2001
We present revised strontium, barium and europium abundances for 63 cool stars
with metallicities [Fe/H] ranging from -2.20 to 0.25. The stellar sample has
been extracted from Fuhrmann's lists ([CITE]). It is
confined to main-sequence and turnoff stars. The results are based on NLTE line
formation obtained in differential model atmosphere analyses of spectra that
have a typical S/N of 200 and a resolution of 40 000 to 60 000. The element
abundance ratios reveal a distinct chemical history of the halo and thick disk
compared with that of the thin disk. Europium is overabundant relative to iron
and barium in halo and thick disk stars suggesting that during the formation of
these galactic populations high-mass stars exploding as SNe II dominated
nucleosynthesis on a short time scale of the order of 1 Gyr. We note the
importance of [Eu/Mg] determinations for halo stars. Our analysis leads to the
preliminary conclusion that Eu/Mg ratios found in halo stars do not support
current theoretical models of the r-process based on low-mass SNe; instead they
seem to point at a halo formation time much shorter than 1 Gyr. A steep decline
of [Eu/Fe] and a slight decline of [Eu/Ba] with increasing metallicity have been
first obtained for thick disk stars. This indicates the start of nucleosynthesis
in the lower mass stars, in SN I and AGB stars, which enriched the interstellar
gas with iron and the most abundant s-process elements. From a decrease of the
Eu/Ba ratio by ~ dex the time interval corresponding to
the thick disk formation phase can be estimated. The step-like change of element
abundance ratios at the thick to thin disk transition found in our previous
analysis (Mashonkina & Gehren [CITE]) is confirmed in this study: [Eu/Ba]
and [Eu/Fe] are reduced by ~0.25 dex and ~0.15 dex, respectively;
[Ba/Fe] increases by ~0.1 dex. This is indicative of an intermediate phase
before the early stage of the thin disk developed, during which only evolved
middle and low mass (<
) stars contributed to nucleosynthesis. Our
data provide an independent method to calculate the duration of this phase. The
main s-process becomes dominant in the production of heavy elements beyond the
iron group during the thin disk evolution. We find that in the thin disk stars
Ba/Fe ratios increase with time from
in stars older than 8 Gyr
to
in stars that are between 2 and 4 Gyr old.
Key words: line: formation / nuclear reactions, nucleosynthesis, abundances / stars: abundances / stars: late-type / Galaxy: evolution
© ESO, 2001
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