Chemical Cartography with APOGEE: Mapping Disk Populations with a
  Two-Process Model and Residual Abundances
  We apply a novel statistical analysis to measurements of 16 elemental
abundances in 34,410 Milky Way disk stars from the final data release (DR17) of
APOGEE-2. Building on recent work, we fit median abundance ratio trends [X/Mg]
vs. [Mg/H] with a 2-process model, which decomposes abundance patterns into a
"prompt" component tracing core collapse supernovae and a "delayed" component
tracing Type Ia supernovae. For each sample star, we fit the amplitudes of
these two components, then compute the residuals \Delta[X/H] from this
two-parameter fit. The rms residuals range from ~0.01-0.03 dex for the most
precisely measured APOGEE abundances to ~0.1 dex for Na, V, and Ce. The
correlations of residuals reveal a complex underlying structure, including a
correlated element group comprised of Ca, Na, Al, K, Cr, and Ce and a separate
group comprised of Ni, V, Mn, and Co. Selecting stars poorly fit by the
2-process model reveals a rich variety of physical outliers and sometimes
subtle measurement errors. Residual abundances allow comparison of populations
controlled for differences in metallicity and [\alpha/Fe]. Relative to the main
disk (R=3-13 kpc, |Z|<2 kpc), we find nearly identical abundance patterns in
the outer disk (R=15-17 kpc), 0.05-0.2 dex depressions of multiple elements in
LMC and Gaia Sausage/Enceladus stars, and wild deviations (0.4-1 dex) of
multiple elements in \omega Cen. Residual abundance analysis opens new
opportunities for discovering chemically distinctive stars and stellar
populations, for empirically constraining nucleosynthetic yields, and for
testing chemical evolution models that include stochasticity in the production
and redistribution of elements.
