Stellar Abundance Maps of the Milky Way Disk

To understand the formation of the Milky Way's prominent bar it is important to know whether stars in the bar differ in the chemical element composition of their birth material as compared to disk stars. This requires stellar abundance measurements for large samples across the Milky Way's body. Such samples, e.g. luminous red giant stars observed by SDSS's Apogee survey, will inevitably span a range of stellar parameters; as a consequence, both modelling imperfections and stellar evolution may preclude consistent and precise estimates of their chemical composition at a level of purported bar signatures, which has left current analyses of a chemically distinct bar inconclusive. Here, we develop a new self-calibration approach to eliminate both modelling and astrophysical abundance systematics among red giant branch (RGB) stars of different luminosities (and hence surface gravity $\log g$). We apply our method to $48,853$ luminous Apogee DR16 RGB stars to construct spatial abundance maps of $20$ chemical elements near the Milky Way's mid-plane, covering Galactocentric radii of $0\,{\rm kpc}<R_{\rm GC}<20\,\rm kpc$. Our results indicate that there are no abundance variations whose geometry matches that of the bar, and that the mean abundance gradients vary smoothly and monotonically with Galactocentric radius. We confirm that the high-$\alpha$ disk is chemically homogeneous, without spatial gradients. Furthermore, we present the most precise [Fe/H] vs. $R_{\rm GC}$ gradient to date with a slope of $-0.057\pm0.001\rm~dex\,kpc^{-1}$ out to approximately $15$ kpc.