Binary blue metal-poor stars: Evidence for asymptotic giant branch mass transfer

We present new abundance analyses of six blue metal- poor (BMP) stars with very low iron abundances ([Fe/H] < -2), based on new high-resolution echelle spectra. Three are spectroscopic binaries, and three have constant radial velocities. The chemical compositions of these two groups are very different, as the binary BMP stars have large enhancements of carbon and neutron-capture elements that are products of s-process nucleosynthesis. One star, CS 29497-030, has an extreme enhancement of lead, [Pb/Fe] = +3.7, the largest abundance in any star yet discovered. It probably also has an oxygen overabundance compared to the other BMP stars of our sample. The binary BMP stars must have attained their status via mass transfer during the asymptotic giant branch (AGB) evolutions of their companion stars, which are now unseen and most likely are compact objects. We have not found any examples of AGB mass transfer among BMP binaries with [Fe/H] > -2.