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

A small set of chemically old stars that appear young by their independently derived masses has been detected. These are so-called a-rich young stars. For a sample of 51 red-giant stars, for which spectra are available from SDSS/APOGEF, and masses are available from asteroseismic measures based on Kepler lightcurves, we derive the C, N, and 0 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 a-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 P/Fel unexplained) or the results of mergers on the main sequence.