Young [$\alpha$/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?

TL;DR

This study discovers young, alpha-enhanced red giant stars in the Milky Way, challenging existing chemical evolution models and suggesting their formation near the Galactic bar's end due to unique gas dynamics.

Contribution

It reports the first identification of young alpha-rich stars in the inner Galactic disc, highlighting their potential formation near the Galactic bar's corotation region, which is a novel insight.

Findings

Young alpha-rich stars are more common in the inner disc.

Their kinematics are not typical of thick disc stars.

These stars likely formed near the Galactic bar's end.

Abstract

We report the discovery of a group of apparently young CoRoT red-giant stars exhibiting enhanced [alpha/Fe] abundance ratios (as determined from APOGEE spectra) with respect to Solar values. Their existence is not explained by standard chemical evolution models of the Milky Way, and shows that the chemical-enrichment history of the Galactic disc is more complex. We find similar stars in previously published samples for which isochrone-ages could be robustly obtained, although in smaller relative numbers, which could explain why these stars have not received prior attention. The young [alpha/Fe]-rich stars are much more numerous in the CoRoT-APOGEE (CoRoGEE) inner-field sample than in any other high-resolution sample available at present, as only CoRoGEE can explore the inner-disc regions and provide ages for its field stars. The kinematic properties of the young [$\alpha$/Fe]-rich stars are not clearly thick-disc like, despite their rather large distances from the Galactic mid-plane. Our tentative interpretation of these and previous intriguing observations in the Milky Way is that these stars were formed close to the end of the Galactic bar, near corotation -- a region where gas can be kept inert for longer times, compared to other regions shocked more frequently by the passage of spiral arms. Moreover, that is where the mass return from older inner-disc stellar generations should be maximal (according to an inside-out disc-formation scenario), further diluting the in-situ gas. Other possibilities to explain these observations (e.g., a recent gas-accretion event) are also discussed.