To Ba or not to Ba: the formation of Barium stars.

It is estimated that about half of the stars in our Galaxy are binaries. Hence, the impact of binarity on several aspects of astronomy is fundamental. Despite of its importance, there are many aspects of binary interaction physics we do not understand yet.

In this research 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 the s-process. The s-process is a neutron nucleosynthesis process which takes place in stellar interiors when a star is on the final stages of its AGB phase. 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. Barium stars are understood as coming from a binary evolution channel in which the former AGB star (now a dimm 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.

With this PhD project, we aim at identifying the initial parameters and the evolutionary channel that will make a binary system to form a Ba star. Additionally the relation of the Ba star systems with its expected progenitors, the AGB and post-AGB systems, is not straightforward, and we also aim at clarifying this aspect of binary evolution. We plan to do this by detailed comparison of evolutionary models, calculated with the state-of-the-art BINSTAR code, with observed orbital and chemical parameters. Our goal is investigating the different possible mass transfer scenarios as well as different eccentricity pumping mechanisms, which are needed to reproduce the observed periods and eccentricities.

In this research 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 the s-process. The s-process is a neutron nucleosynthesis process which takes place in stellar interiors when a star is on the final stages of its AGB phase. 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. Barium stars are understood as coming from a binary evolution channel in which the former AGB star (now a dimm 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.

With this PhD project, we aim at identifying the initial parameters and the evolutionary channel that will make a binary system to form a Ba star. Additionally the relation of the Ba star systems with its expected progenitors, the AGB and post-AGB systems, is not straightforward, and we also aim at clarifying this aspect of binary evolution. We plan to do this by detailed comparison of evolutionary models, calculated with the state-of-the-art BINSTAR code, with observed orbital and chemical parameters. Our goal is investigating the different possible mass transfer scenarios as well as different eccentricity pumping mechanisms, which are needed to reproduce the observed periods and eccentricities.