The Metal-weak Milky Way Stellar Disk Hidden in the Gaia-Sausage-Enceladus Debris: The APOGEE DR17 View

Abstract

We have for the first time identified the early stellar disk in the Milky Way by using a combination of elemental abundances and kinematics. Using data from APOGEE DR17 and Gaia we select stars in the Mg-Mn-Al-Fe plane with elemental abundances indicative of an accreted origin and find stars with both halo-like and disk-like kinematics. The stars with halo-like kinematics lie along a lower sequence in [Mg/Fe], while the stars with disk-like kinematics lie along a higher sequence. Combined with astroseismic observations, we determine that the stars with halo-like kinematics are old, 9-11 Gyr, and that the more evolved stellar disk is about 1-2 Gyr younger. We show that the in situ fraction of stars on deeply bound orbits is not small, in fact the inner Galaxy likely harbors a genuine in situ population together with an accreted one. In addition, we show that the selection of the Gaia-Sausage-Enceladus in the E _n-L _z plane is not very robust. In fact, radically different selection criteria give almost identical elemental abundance signatures for the accreted stars.