High- and Low-$\alpha$ Disk Stars Separate Dynamically at all Ages
  There is a dichotomy in the Milky Way in the $[\alpha/$Fe]-[Fe/H] plane, in
which stars fall into high-$\alpha$, and low-$\alpha$ sequences. The
high-$\alpha$ sequence comprises mostly old stars, and the low-$\alpha$
sequence comprises primarily young stars. The origin of this dichotomy is
uncertain. To better understand how the high- and low-$\alpha$ stars are
affiliated, we examine if the high- and low-$\alpha$ sequences have distinct
orbits at all ages, or if age sets the orbital properties of stars irrespective
of their $\alpha$-enhancement. Orbital actions $J_R$, $J_z$, and $J_\phi$ (or
$L_z$) are our labels of stellar dynamics. We use ages for 58,278 LAMOST stars
(measured to a precision of 40\%) within $\leq$2kpc of the Sun and we calculate
orbital actions from proper motions and parallaxes given by Gaia's DR2. We find
that \emph{at all ages}, the high- and low-$\alpha$ sequences are dynamically
distinct. This implies separate formation and evolutionary histories for the
two sequences; a star's membership in the high- or low-$\alpha$ sequence
indicates its dynamical properties at a given time. We use action space to make
an efficient selection of halo stars and subsequently report a group of old,
low-$\alpha$ stars in the halo, which may be a discrete population from an
infall event.
