High- and Low-α Disk Stars Separate Dynamically at All Ages

There is a dichotomy in the Milky Way in the [ α /Fe]–[Fe/H] plane, in which stars fall into high- α and low- α sequences. The high- α sequence comprises mostly old stars, and the low- α sequence comprises primarily young stars. The origin of this dichotomy is uncertain. To better understand how the high- and low- α stars are affiliated, we examine if the high- and low- α sequences have distinct orbits at all ages, or if age sets the orbital properties of stars irrespective of their α -enhancement. Orbital actions J R , J z , and J ϕ (or L z ) are our labels of stellar dynamics. We use ages for 58,278 Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) stars (measured to a precision of 40%) within ≤2 kpc of the Sun and we calculate orbital actions from proper motions and parallaxes given by Gaia ’s DR2. We find that at all ages , the high- and low- α sequences are dynamically distinct. This implies separate formation and evolutionary histories for the two sequences; a star’s membership in the high- or low- α 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- α stars in the halo, which may be a discrete population from an infall event.