Origin of $\alpha$-rich young stars: clues from C, N and O

Abstract

A small set of chemically old stars that appear young by their independently derived masses has been detected. These are so-called $\alpha$-rich young stars. For a sample of 51 red-giant stars, for which spectra are available from SDSS/APOGEE and masses are available from asteroseismic measures based on \textit{Kepler} lightcurves, we derive the C, N and O abundances through an independent analysis. These stars span a wide range of N/C surface number density ratios. We interpret the high-mass stars with low N/C as being products of mergers or mass transfer during or after first dredge up, because the dredge-up features are the same as for low-mass stars. The $\alpha$-rich young stars with high N/C follow the expected trend of N/C for their mass, and could be either genuine young stars (leaving their high [$\alpha$/Fe] unexplained) or the results of mergers on the main sequence.