# The metal-weak Milky Way stellar disk hidden in the Gaia-Sausage-Enceladus debris: the APOGEE DR17 view

## TL;DR

This study uncovers the ancient stellar disk in the Milky Way by combining elemental abundances and kinematics from APOGEE DR17 and Gaia data, revealing a hidden metal-weak disk component and its properties.

## Contribution

It is the first to identify the early stellar disk in the Milky Way using combined elemental and kinematic data, highlighting the presence of in-situ stars alongside accreted populations.

## Key findings

- Old halo-like stars are 9-11 Gyr old.
- The stellar disk is about 1-2 Gyr younger than halo stars.
- Selection criteria for Gaia-Sausage-Enceladus are not very robust.

## 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 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. Through with asteroseismic observations, we determine 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 harbours a genuine in-situ population together with an accreted one. In addition, we show that the selection of Gaia-Sausage-Enceladus in the En-Lz plane is not very robust. In fact, radically different selection criteria give almost identical elemental abundance signatures for the accreted stars.