Cannibals in the thick disk: the young $\alpha-$rich stars as evolved blue stragglers

TL;DR

This study suggests that some young, alpha-rich thick disk stars are likely evolved blue stragglers formed through binary mass transfer, supported by radial velocity and chemical abundance analyses.

Contribution

It provides evidence that certain young alpha-rich stars are evolved blue stragglers resulting from binary evolution, challenging previous age-based classifications.

Findings

More 'young' stars are in binary systems compared to 'old' stars.

'Young' stars do not follow expected [C/H] versus mass trends.

Population synthesis reproduces observed [C/N] ratios versus mass.

Abstract

Spectro-seismic measurements of red giants enabled the recent discovery of stars in the thick disk that are more massive than 1.4 M_sun. While it has been claimed that most of these stars are younger than the rest of the typical thick disk stars, we show evidence that they might be products of mass transfer in binary evolution, notably evolved blue stragglers. We took new measurements of the radial velocities in a sample of 26 stars from APOKASC, including 13 "young" stars and 13 "old" stars with similar stellar parameters but with masses below 1.2 M_sun and found that more of the "young" stars appear to be in binary systems with respect to the "old" stars. Furthermore, we show that the "young" stars do not follow the expected trend of [C/H] ratios versus mass for individual stars. However, with a population synthesis of low-mass stars including binary evolution and mass transfer, we can reproduce the observed [C/N] ratios versus mass. Our study shows how asteroseismology of solar-type red giants provides us with a unique opportunity to study the evolution of field blue stragglers after they have left the main-sequence.