Binary stars in the Galactic thick disc
Binary stars in the Galactic thick disc
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DOI：
10.1093/mnras/stx2355
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发表时间：
2017-09
影响因子：
4.8
通讯作者：
R. Izzard;H. Preece;P. Jofré;Ghina M. Halabi;T. Masseron;C. Tout
中科院分区：
文献类型：
--
作者：
R. Izzard;H. Preece;P. Jofré;Ghina M. Halabi;T. Masseron;C. Tout
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.
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.