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 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.

KELT-21b: A Hot Jupiter Transiting the Rapidly Rotating Metal-poor Late-A Primary of a Likely Hierarchical Triple System

“…For the latter sample, we selected stars with Galactic periapses 6.6 kpc > and apoapses 10.0 kpc < in order to produce a sample with similar kinematics to KELT-21. For both samples, we excluded stars with [C/N]>−0.4 in order to exclude stars that are likely merger products or have experienced significant mass gain due to binary interactions, and are therefore likely older than would be assumed given their other properties (cf., Izzard et al 2018 Figure 13. It is apparent that KELT-21ʼs Fe H [ ] is at the lower end of these distributions, but it is not a dramatic outlier.…”

Origin of α-rich young stars: clues from C, N, and O

“…Furthermore, Tayar et al (2015) found that one of these stars was rapidly rotating, another sign that an interaction could be involved. Together with the theoretical work by Izzard et al (2018) there is mounting evidence that at least a significant part of the massive, i.e. young α-rich stars is a result of binary interactions.…”

“…These interactions can either produce a higher mass star in a binary through mass transfer, or a more massive single star through a merger. However, one needs to note that the main conclusions of Izzard et al (2018) are a prediction that the fraction of thick disk giants with mass in excess of 1.3 M ⊙ ranges from 0.8 to 3 percent in all but one of their model sets. In contrast, Martig et al (2015) found ∼6% of their α-rich stars were high mass.…”

“…Only when Izzard et al (2018) change from a log-normal distribution of orbital separations to a (less representative) logarithmically-flat separation distribution do they predict a fraction of giants with mass in excess of 1.3 M ⊙ to be large at 11%. Izzard et al (2018) also included an analysis of [C/N] in their analysis of theoretical models. In these models they boosted their initial [C/N] by +0.2 dex to mimic chemical evolution in the thick disk.…”

Carbon-deficient Red Giants

“…The above discussions have considered the evolution of single stars. In a recent paper, Izzard et al (2018) discuss mass-transfer and stellar mergers, in the context of explaining the presence of fairly massive stars in the old population of the Galactic thick disk. They raise the possibility that C-poor and N-rich stars could be the result of binary-star interactions that expose CN-processed material at the surface, such as in the CDRGs.…”