Institute of Astronomy

To Ba or not to Ba: the Formation of Barium Stars

SpeakerTalk DateTalk Series
Ana Escorza29 July 2016Binary Stars Talks


In this contribution we will focus on the formation of chemically peculiar stars known as Barium stars. These stars have an excess of barium on their surface as well as several other chemical elements which are products of slow-neutron-capture nucleosynthesis (s-process). The presence of s-process elements on the surface of a main-sequence or a giant star cannot be explained by single star evolutionary theory, and Barium stars are understood as coming from a binary evolution channel in which the former AGB star (now a dim white dwarf) polluted the current Ba star with enriched products via a mass transfer process. A long-standing problem is, however, that the observed orbital characteristics of Ba stars are not predicted by the current binary evolution models. Important problems remain concerning in particular the persistence of high eccentricity in systems with relatively short periods. We will discuss the results of our on-going radial velocity programme and compare the orbital distribution of Ba dwarfs and Ba giants. We aim at identifying the initial parameters and the evolutionary channel that will make a binary system to form a Ba star. We will present the result of our first evolutionary models, calculated with the state-of-the-art BINSTAR code. Our goal is to investigate different possible mass-transfer scenarios as well as different eccentricity pumping mechanisms, which are needed to reproduce the observed periods and eccentricities, for both Ba dwarfs and Ba giants.