Binary stars in the Galactic thick disc
Robert G. Izzard, Holly Preece, Paula Jofre, Ghina M. Halabi, Thomas, Masseron, Christopher A. Tout

TL;DR
This paper investigates why some ancient thick-disc stars appear more massive than expected by modeling binary interactions, such as mass transfer and merging, to explain their increased masses and surface abundances.
Contribution
It introduces a binary population-nucleosynthesis model to explain the presence of unexpectedly massive thick-disc stars, emphasizing the role of binary interactions.
Findings
A few percent of thick-disc stars are affected by binary interactions.
Most massive stars are merged binaries, explaining their single status.
Model results align with APOKASC data when considering binary fraction and orbital-period distribution.
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 . So why do the masses of a sizeable fraction of thick-disc stars exceed , with some as massive as ? 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…
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