Modelling Blue Straggler Populations with BPASS

Abstract

Most binary population synthesis models fail to predict Blue Straggler populations accurately, largely underpredicting the ratio of Blue Stragglers to Red Giants. Binary interaction and masstransfer are key processes in the formation of Blue Stragglers and therefore they are a sensitive test for the mass-transfer stability of binary models. This thesis tests predictions by BPASS (Binary Population and Spectral Synthesis) models for Blue Straggler populations. Results show that binary systems with primary stars in the 1M⊙ to 2M⊙ mass range are the major contributors to the increase of Blue Stragglers from log(age/yrs)=9 to log(age/yrs)=10. A comparison of observational data to BPASS models shows agreement with Blue Stragglers/Red Giants ratio within the margin of error, much closer than other population synthesis models. We find that this is due to the stellar structure evolving in detail throughout the mass-transfer. This results in more stable mass-transfer events than is estimated with rapid evolution codes. Understanding mass transfer in these low mass stars is important for a variety of aspects of astrophysics, for example, in the formation of type Ia supernovae through the single-degenerate scenario. We suggest the next step in this project would be to explore this pathway as well as study a greater sample of old stellar populations to constrain the models of mass transfer to a greater degree.