Binary stars in the Galactic thick disc

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.