The formation of barium giants via mass accretion in binary systems

We examine the composition of barium stars in the context of mass transfer from an asymptotic giant branch (AGB) companion. We accrete between 0.01 and 0.5M(circle dot) of AGB ejecta on to low-mass companions of [Fe/H] = -0.25 at the ages expected for the end of the lives of AGB stars of 2.5, 3, and 4M(circle dot). In each case, we form a star of 2.5M(circle dot) that is thought to be a typical barium star mass. We discuss the extent of dilution of accreted material as the star evolves, and describe the impact on the surface abundances. For accretion from a 2.5M(circle dot) primary, if the secondary's initial mass is 2.45M(circle dot) or more, accretion takes place when the secondary is undergoing core helium burning. Using data from the sample of De Castro et al., we attempt to fit the observed properties of 74 barium giants using the models we have computed. We find that all but six of these objects are best fit using ejecta from 2.5M(circle dot) (32 objects) or 3M(circle dot) (36 objects) AGB stars. Higher accretion masses are typically required when accreting from a lower mass companion. We find accretion masses that are broadly consistent with recent hydrodynamical simulations of wind mass transfer, though the accretion efficiency is towards the upper limit found in these simulations. For the 18 stars with reported orbital periods, we find no strong correlations between period and accretion mass.

Automated summary

We examine the composition of barium stars in the context of mass transfer from an asymptotic giant branch (AGB) companion. Accretion of AGB ejecta onto low-mass companions forms typical 2.5M barium stars, with higher accretion masses required for lower mass companions. Our models fit observed properties of 74 barium giants, with most objects best fit with ejecta from 2.5M or 3M AGB stars. Accretion efficiency is towards the upper limit found in hydrodynamical simulations, with no strong correlations found between orbital period and accretion mass for the 18 stars with reported periods.