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.

UVO 0825+15 is a hot bright helium-rich subdwarf which lies in K2 Field 5 and in a sample of intermediate helium-rich subdwarfs observed with Subaru/HDS. The K2 light curve shows low-amplitude variations, whilst the Subaru spectrum shows Pbiv absorption lines, indicative of a very high lead overabundance. UVO 0825+15 also has a high proper motion with kinematics typical for a thick disk star. Analyses of ultraviolet and intermediate dispersion optical spectra rule out a short-period binary companion, and provide fundamental atmospheric parameters of T eff = 38 900±270 K, log g/cm s −2 = 5.97 ± 0.11, log n He /n H = −0.57 ± 0.01, E B−V ≈ 0.03, and angular radius θ = 1.062±0.006×10−11 radians (formal errors). The high-resolution spectrum shows that carbon is > 2 dex subsolar, iron is approximately solar and all other elements heavier than argon are at least 2 -4 dex overabundant, including germanium, yttrium and lead. Approximately 150 lines in the blue-optical spectrum remain unidentified. The chemical structure of the photosphere is presumed to be determined by radiatively-dominated diffusion. The K2 light curve shows a dominant period around 10.8 h, with a variable amplitude, its first harmonic, and another period at 13.3 h. The preferred explanation is multi-periodic non-radial oscillation due to g-modes with very high radial order, although this presents difficulties for pulsation theory. Alternative explanations fail for lack of radial-velocity evidence. UVO 0825+15 represents the fourth member of a group of hot subdwarfs having helium-enriched photospheres and 3-4 dex overabundances of trans-iron elements, and is the first lead-rich subdwarf to show evidence of pulsations.

In recent years, observations have shown that multiple-star systems such as hierarchical triple and quadruple-star systems are common, especially among massive stars. They are potential sources of interesting astrophysical phenomena such as compact object mergers, leading to supernovae, and gravitational wave events. However, many uncertainties remain in their often complex evolution. Here, we present the population synthesis code Multiple Stellar Evolution (mse), designed to rapidly model the stellar, binary, and dynamical evolution of multiple-star systems. mse includes a number of new features not present in previous population synthesis codes: 1) an arbitrary number of stars, as long as the initial system is hierarchical, 2) dynamic switching between secular and direct N-body integration for efficient computation of the gravitational dynamics, 3) treatment of mass transfer in eccentric orbits, which occurs commonly in multiple-star systems, 4) a simple treatment of tidal, common-envelope, and mass transfer evolution in which the accretor is a binary instead of a single star, 5) taking into account planets within the stellar system, and 6) including gravitational perturbations from passing field stars. mse, written primarily in the c++ language, will be made publicly available and has few prerequisites; a convenient Python interface is provided. We give a detailed description of MSE and illustrate how to use the code in practice. We demonstrate its operation in a number of examples.