Binary stars in the Galactic thick disc

Abstract

The combination of asteroseismologically-measured masses with abundances from detailed analyses of stellar atmospheres challenges our fundamental knowledge of stars and our ability to model them. Ancient red-giant stars in the Galactic thick disc are proving to be most troublesome in this regard. They are older than 5 Gyr, a lifetime corresponding to an initial stellar mass of about $1.2{\mathrm{M}}_{\odot }$. So why do the masses of a sizeable fraction of thick-disc stars exceed $1.3{\mathrm{M}}_{\odot }$, with some as massive as $2.3{\mathrm{M}}_{\odot }$ ? We answer this question by considering duplicity in the thick-disc stellar population using a binary population-nucleosynthesis model. We examine how mass transfer and merging affect the stellar mass distribution and surface abundances of carbon and nitrogen. We show that a few per cent of thick-disc stars can interact in binary star systems and become more massive than $1.3{\mathrm{M}}_{\odot }$. Of these stars, most are single because they are merged binaries. Some stars more massive than $1.3{\mathrm{M}}_{\odot }$ form in binaries by wind mass transfer. We compare our results to a sample of the APOKASC data set and find reasonable agreement except in the number of these thick-disc stars more massive than $1.3{\mathrm{M}}_{\odot }$. This problem is resolved by the use of a logarithmically-flat orbital-period distribution and a large binary fraction.

This is a pre-copyedited, author-produced version of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record [Robert Izzard, 'Binary stars in the Galactic thick disc', Monthly Notices of the Royal Astronomical Society, accepted 7th Sep. 2017] is at https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stx2355

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Cited by in corpus (1)

• Origin of $\alpha$-rich young stars: clues from C, N and O