The Case for S Star Binaries

Abstract

Several lines of evidence point to a scenario in which Tc-poor S stars are the cooler analogs to the Ba II stars, i.e. they are binary systems where the peculiar atmospheric composition of the primary star is due to mass accreted from a secondary star long ago. Cases have been found where such S stars have WD or main-sequence companions, but an increasing number are found to be in interactive, symbiotic-like systems. Evidence of wind shocks, gas-streaming, and/or accretion disks in these systems attest to the current proximity of the components and provide striking evidence that accretion can be an important mechanism in their chemical evolution. Interactive effects are not as prominent in Ba II stars, presumably because of the lack of a strong wind from the primary star.