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.

Candidate Members of the VMP/EMP Disk System of the Galaxy from the SkyMapper and SAGES Surveys

“…The best way to distinguish between these two possibilities may be to conduct a thorough study of their elemental abundances and look for differences as a function of declining metallicity. Feltzing & Feuillet (2023) have recently used elemental abundance information from APOGEE, in combination with kinematics, in order to identify the likely presence of an early disk structure in the inner disk of the MW including VMP stars (although they are limited by the lack of lowermetallicity stars in APOGEE, precluding verification that EMP stars are present as well). Detailed chemical abundances for our candidate VMP/EMP stars would clearly be useful.…”