Most "young" $\alpha$-rich stars have high masses but are actually old

Abstract

Recent observations have revealed a population of $\alpha$-element abundances enhanced giant stars with unexpected high masses ($\gtrsim$1 $M_\odot$) from asteroseismic analysis and spectroscopy. Assuming single-star evolution, their masses imply young ages ($\tau<6$Gyr) incompatible with the canonical Galactic chemical evolution scenario. Here we study the chemistry and kinematics of a large sample of such $\alpha$-rich, high-mass red giant branch (RGB) stars drawn from the LAMOST spectroscopic surveys. Using LAMOST and Gaia, we found these stars share the same kinematics as the canonical high-$\alpha$ old stellar population in the Galactic thick disk. The stellar abundances show that these high-$\alpha$ massive stars have $\alpha$- and iron-peak element abundances similar to those of the high-$\alpha$ old thick disk stars. However, a portion of them exhibit higher [(N+C)/Fe] and [Ba/Fe] ratios, which implies they have gained C- and Ba-rich materials from extra sources, presumably asymptotic giant branch (AGB) companions. The results support the previous suggestion that these RGB stars are products of binary evolution. Their high masses thus mimic "young" single stars, yet in fact they belong to an intrinsic old stellar population. To fully explain the stellar abundance patterns of our sample stars, a variety of binary evolution channels, such as, main-sequence (MS) + RGB, MS + AGB, RGB + RGB and RGB + AGB, are required, pointing to diverse formation mechanisms of these seemly rejuvenated cannibals. With this larger sample, our results confirm earlier findings that most, if not all, $\alpha$-rich stars in the Galactic disk seem to be old.