First Direct Evidence That Barium Dwarfs Have White Dwarf Companions

Details First Direct Evidence That Barium Dwarfs Have White Dwarf Companions First Direct Evidence That Barium Dwarfs Have White Dwarf Companions

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aj....141..160g&link_type=abstract

The Astronomical Journal, Volume 141, Issue 5, article id. 160 (2011).

Astronomy and Astrophysics

Astronomy

1

Binaries: Spectroscopic, Stars: Chemically Peculiar, Stars: Evolution, Stars: Individual: Hd 2454 Hd 15306 Hd 26367 Hd 34654 Hd 114520 Hd 221531, White Dwarfs

Scientific paper

Barium II (Ba) stars are chemically peculiar F-, G-, and K-type objects that show enhanced abundances of s-process elements. Since s-process nucleosynthesis is unlikely to take place in stars prior to the advanced asymptotic giant branch (AGB) stage, the prevailing hypothesis is that each present Ba star was contaminated by an AGB companion which is now a white dwarf (WD). Unless the initial mass ratio of such a binary was fairly close to unity, the receiving star is thus at least as likely to be a dwarf as a giant. So although most known Ba stars appear to be giants, the hypothesis requires that Ba dwarfs be comparably plentiful and moreover that they should all have WD companions. However, despite dedicated searches with the IUE satellite, no WD companions have been directly detected to date among the classical Ba dwarfs, even though some 90% of those stars are spectroscopic binaries, so the contamination hypothesis is therefore presently in some jeopardy. In this paper, we analyze recent deep, near-UV and far-UV Galaxy Evolution Explorer (GALEX) exposures of four of the brightest of the class (HD 2454, 15360, 26367, and 221531), together with archived GALEX data for two newly recognized Ba dwarfs: HD 34654 and HD 114520 (which also prove to be spectroscopic binaries). The GALEX observations of the Ba dwarfs as a group show a significant far-UV excess compared to a control sample of normal F-type dwarfs. We suggest that this ensemble far-UV excess constitutes the first direct evidence that Ba dwarfs have WD companions.

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