Cannibals in the thick disk II -- Radial-velocity monitoring of the young alpha-rich stars

TL;DR

This study investigates young alpha-rich stars in the Milky Way's thick disk using radial velocity, spectroscopy, and astrometry to understand their ages, binarity, and origins, revealing many are likely products of mass transfer.

Contribution

It provides new observational evidence on the binarity, mass, and chemical properties of young alpha-rich stars, supporting the hypothesis that many are formed through mass transfer events.

Findings

Stars with mass < 1 M_sun are all single.

Stars with mass > 1 M_sun can be single or binary.

Many YAR stars show chemical trends indicating mass transfer origins.

Abstract

Determining ages of stars for reconstructing the history of the Milky Way remains one of the most difficult tasks in astrophysics. This involves knowing when it is possible to relate the stellar mass with its age and when it is not. The young $\alpha-$rich (YAR) stars present such a case in which we are still not sure about their ages because they are relatively massive, implying young ages, but their abundances are $\alpha-$enhanced, which implies old ages. We report the results from new observations from a long-term radial-velocity-monitoring campaign complemented with high-resolution spectroscopy, as well as new astrometry and seismology of a sample of 41 red giants from the third version of APOKASC, which includes YAR stars. The aim is to better characterize the YAR stars in terms of binarity, mass, abundance trends, and kinematic properties.The radial velocities of HERMES, APOGEE, and Gaia were combined to determine the binary fraction among YAR stars. In combination with their mass estimate, evolutionary status, chemical composition, and kinematic properties, it allowed us to better constrain the nature of these objects. We found that stars with $\mathrm{M} < 1 \mathrm{M}_\odot$ were all single, whereas stars with $\mathrm{M} > 1 \mathrm{M}_\odot$ could be either single or binary. This is in agreement with theoretical predictions of population synthesis models. Studying their [C/N], [C/Fe], and [N/Fe], trends with mass, it became clear that many YAR stars do not follow the APOKASC stars, favoring the scenario that most of them are the product of mass transfer. Abr.