Binary Blue Metal‐poor Stars: Evidence for Asymptotic Giant Branch Mass Transfer

Abstract

We present new abundance analyses of six blue metal-poor (BMP) starswith very low iron abundances ([Fe/H]{\lt}-2), based on newhigh-resolution echelle spectra. Three are spectroscopic binaries, andthree have constant radial velocities. The chemical compositions ofthese two groups are very different, as the binary BMP stars have largeenhancements of carbon and neutron-capture elements that are products ofs-process nucleosynthesis. One star, CS 29497-030, has an extremeenhancement of lead, [Pb/Fe]=+3.7, the largest abundance in any star yetdiscovered. It probably also has an oxygen overabundance compared to theother BMP stars of our sample. The binary BMP stars must have attainedtheir status via mass transfer during the asymptotic giant branch (AGB)evolutions of their companion stars, which are now unseen and mostlikely are compact objects. We have not found any examples of AGB masstransfer among BMP binaries with [Fe/H]{\gt}-2.