Most "young" $\alpha$-rich stars have high masses but are actually old
  Recent observations have revealed a population of $\alpha$-element abundances
enhanced giant stars with unexpected high masses ($\gtrsim$1 $M_\odot$) from
asteroseismic analysis and spectroscopy. Assuming single-star evolution, their
masses imply young ages ($\tau<6$Gyr) incompatible with the canonical Galactic
chemical evolution scenario. Here we study the chemistry and kinematics of a
large sample of such $\alpha$-rich, high-mass red giant branch (RGB) stars
drawn from the LAMOST spectroscopic surveys. Using LAMOST and Gaia, we found
these stars share the same kinematics as the canonical high-$\alpha$ old
stellar population in the Galactic thick disk. The stellar abundances show that
these high-$\alpha$ massive stars have $\alpha$- and iron-peak element
abundances similar to those of the high-$\alpha$ old thick disk stars. However,
a portion of them exhibit higher [(N+C)/Fe] and [Ba/Fe] ratios, which implies
they have gained C- and Ba-rich materials from extra sources, presumably
asymptotic giant branch (AGB) companions. The results support the previous
suggestion that these RGB stars are products of binary evolution. Their high
masses thus mimic "young" single stars, yet in fact they belong to an intrinsic
old stellar population. To fully explain the stellar abundance patterns of our
sample stars, a variety of binary evolution channels, such as, main-sequence
(MS) + RGB, MS + AGB, RGB + RGB and RGB + AGB, are required, pointing to
diverse formation mechanisms of these seemly rejuvenated cannibals. With this
larger sample, our results confirm earlier findings that most, if not all,
$\alpha$-rich stars in the Galactic disk seem to be old.
