Evolved stars with inconsistent age estimates: Abundance outliers or mass transfer products?

Abstract

In the Milky Way disk there is a strong trend linking stellar age to surface element abundances. Here we explore this relationship with a dataset of 8,803 red-giant and red-clump stars with both asteroseismic data from NASA Kepler Mission and surface abundances from the SDSS-V MWM. We find, with a k-nearest-neighbors approach, that the [Mg/H] and [Fe/Mg] abundance ratios predict asteroseismic ages to an accuracy of about 2 Gyr for the majority of stars. That said, there are substantial outlier stars whose surface abundances do not match their asteroseismic ages. Because asteroseismic ages for these stars are fundamentally based on density or mass, these outliers are mass-transfer candidates. Stars whose surface abundances predict a younger age (higher mass) than what's seen in the asteroseismology are mass accretor candidates (MAC); stars whose abundances predict an older age (lower mass) than the asteroseismology age are mass donor candidates (MDC). We create precise control samples, matched according to (1) surface abundances and (2) asteroseismic ages, for both the MAC and MDC stars; we use these to find slight differences in rotational velocity, [C/N], and [Na/Mg] between the mass-transfer candidates and their abundance neighbors. We find no drastic differences in kinematics, orbital invariants, UV excess, or other stellar abundances between outliers and their abundance neighbors. We deliver 377 mass-transfer candidates for follow-up observations. This project implicitly suggests a fundamental limit on the reliability of asteroseismic ages, and supports existing evidence that age-abundance outliers are products of binary mass transfer.