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.