New Evidence of Binarity in Young {\alpha}-Rich Turn-off and Subgiant Stars: Fast Rotation and Strong Magnetic Activity

TL;DR

This study provides evidence that young alpha-rich stars are often fast rotators with magnetic activity, supporting the idea that they result from binary evolution processes like mass transfer or mergers.

Contribution

It demonstrates that young alpha-rich stars exhibit rapid rotation and magnetic activity, linking these features to binary evolution scenarios.

Findings

YAR stars are fast rotators and magnetically active.

Some YAR stars show signs of past mass transfer from companions.

Magnetic activity can be used to probe binary evolution in peculiar stars.

Abstract

Young {\alpha}-rich (YAR) stars within the old Galactic thick disk exhibit a dual characteristic of relative youth determined with asteroseismology and abundance enhancement in {\alpha} elements measured from high-resolution spectroscopy. The youth origin of YAR stars has been proposed to be binary evolution via mass transfer or stellar mergers. If that is the case, YAR stars should spin rapidly and thus be magnetically active, because they are mass and angular momentum gainers. In this study, to seek this binary footprint we select YAR stars on the main-sequence turn-off or the subgiant branch (MSTO-SGB) from APOGEE DR17, whose ages and projected rotation velocities (vsini) can be precisely measured. With APOGEE vsini and LAMOST spectra, we find that YAR stars are indeed fast rotators and magnetically active. In addition, we observe low [C/N] ratios and high Gaia RUWE in some YAR stars, suggesting that these MSTO-SGB stars probably have experienced mass transfer from red-giant companions. Our findings underscore that magnetic activity can serve as a valuable tool for probing the binary evolution for other chemically peculiar stars, such as red giants with lithium anomalies and carbon-enhanced metal-poor stars.