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We study the possibility that the Milky Way's cool stellar disc includesmergers with ancient stars. Galaxies are understood to form in a hierarchicalmanner, where smaller (proto-)galaxies merge into larger ones. Stars ingalaxies, like the Milky Way, contain in their motions and elemental abundancestracers of past events and can be used to disentangle merger remnants fromstars that formed in the main galaxy. The merger history of the Milky Way isgenerally understood to be particularly easy to study in the stellar halo. Theadvent of the ESA astrometric satellite Gaia has enabled the detection ofcompletely new structures in the halo such as the Gaia-Enceladus-Sausage.However, simulations also show that mergers may be important for the build-upof the cool stellar disks. Combining elemental abundances for 100 giant branchstars from APOGEE DR17 and astrometric data from Gaia we use elementalabundance ratios to find an hitherto unknown, old stellar component in the coolstellar disk in the Milky Way. We further identify a small sample of RR Lyraevariables with disk kinematics that also show the same chemical signature asthe accreted red giant stars in the disk. These stars allows us to date thestars in the accreted component. We find that they are exclusively old.

An old, metal-rich accreted stellar component in the Milky Way stellar disk