Barium stars and their white-dwarf companions, a key to understanding evolved binaries

Abstract

A rich zoo of peculiar objects forms when evolved stars with extended and loosely-bound convective envelopes, such as Asymptotic Giant Branch (AGB) stars, undergo gravitational interaction in binary systems. For example, Barium (Ba) stars are main-sequence and red-giant stars that accreted mass from the outflows of a former AGB companion. This companion is now a dim and, in most cases, not directly detectable, white dwarf (WD). The orbital properties of barium stars can help us constrain interaction mechanisms in binary systems with giant components, and their chemical abundances are a tracer of the nucleosynthesis processes that took place inside the former AGB star. This contribution presents the most recent observational constraints concerning the orbital and stellar properties of Ba stars, which have increased in quantity and quality in the past few years thanks to long-term radial-velocity monitoring programs and to the accurate distances provided by the Gaia mission. However, until recently, important uncertainties affected the properties of their faint white dwarf companions, which contain key information about the formation of Ba stars. Combining radial-velocity data with Hipparcos and Gaia astrometry, we accurately measured the orbital inclinations of these binary systems and obtained the absolute masses of these otherwise hidden white dwarfs. The stellar and orbital properties of Ba stars, including the WD companion masses, are essential for our understanding of these systems and are important input parameters for binary evolution and AGB nucleosynthesis models.