Binary stars in the Galactic thick disc. Issue 3 (15th September 2017)

• Record Type: Journal Article
• Authors: Izzard, Robert G.; Preece, Holly; Jofre, Paula; Halabi, Ghina M.; Masseron, Thomas; Tout, Christopher A.
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 473:Issue 3(2018)
• Page Start: 2984
• Page End: 2999
• Publication Date: 2017-09-15
• Subjects: binaries: general -- Galaxy: disc -- Galaxy: stellar content; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stx2355
• Languages: English
• ISSNs: 0035-8711

Abstract

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 M⊙ . So why do the masses of a sizeable fraction of thick-disc stars exceed 1.3 M⊙, with some as massive as 2.3 M⊙ ? 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 M⊙ . Of these stars, most are single because they are merged binaries. Some stars more massive than 1.3 M⊙ 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 M⊙ . This problem is resolved by the use of a logarithmically flat orbital-period distribution and a large binary fraction.