Abstract

Recent studies of old stars in the Galactic thick disc suggest it contains a population of “young” giant stars. These are chemically and kinematically members of the thick disc, but their masses – measured by asteroseismology – are up to a factor of two greater than expected given the age of the thick disc. Galactic astronomers have suggested they come from a population of stars that have migrated from the bar of the Galaxy. Because migration away from a bar is anathema to us, we show that they could instead be (ex-?)binary stars. These stars have undergone mass transfer, a process similar to that which forms blue stragglers in globular clusters, or they have merged. Through the carbon–to–nitrogen ratio we relate stellar surface chemistry to stellar mass using single and binary star models. The latter match the observations far better than we could have expected. We also predict binary fractions and other properties of these “young” stars ready for comparison with ongoing and upcoming observational programmes.