  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.
