Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...283..937z&link_type=abstract
Astronomy and Astrophysics, Vol. 283, NO. 3/MAR(III), P. 937, 1994
Astronomy and Astrophysics
Astrophysics
31
Stars: Abundances Stars: Chemically Peculiar - Stars: Atmospheres - Stars: Evolution - Nucleosynthesis
Scientific paper
Detailed abundance analyses have been carried out for 31 barium and normal G-K giant stars using high-dispersion spectra and model atmospheres. A significant enhancement of s-process elements is found for seventeen stars. The abundances of light and iron-peak elements are in general equal to those in the standard star ɛ Vir. However, Na, Mg, Mn, and Co are systematically slightly deficient (about 0.2 dex). The elements heavier than Ni are enhanced by up to about 1.5 dex compared with the standards, while the r-process element Eu abundance is roughly normal. We cannot find substantial differences in abundances, atmospheric parameters, and luminosities of radial- velocity variable and nonvariable barium stars. Therefore it seems that both groups of stars belong to a single family of peculiar giants.
Comparison between the mean observed s-process abundances for our uniform barium star sample, and theoretical predictions from various neutron exposure models, show that 13C neutron source AGB star (with the mean neutron exposure τ0 > 0.4mb-1 can best reproduce the abundance data of these stars. Low neutron density single neutron exposures of approx. 1.1mb-1 also are shown to result in good agreement with the barium star observations.
Mass transfer scenarios are tested using the chemical composition and orbital parameter data of Ba II stars. Since a correlation exists between s-process abundance anomalies and orbital periods (projections of the semi-major axis, mass functions) for barium star binaries, we conclude that a wind accretion scenario is more promising than Roche-lobe overflow ones.
Abundance patterns for barium and carbon stars have been compared. We find good agreement for the iron group metals, but carbon stars show higher s-process element overabundances (0.9 dex in the mean). Therefore, the companions to the Ba II stars were perhaps once carbon stars, who overflowed mass onto the presently visible star with a second dilution (the ratio of the transferred mass to the mass of the receiving envelope) of roughly 0.9.
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