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.
