Residual Abundances in GALAH DR3: Implications for Nucleosynthesis and
  Identification of Unique Stellar Populations
  We investigate the [X/Mg] abundances of 16 elements for 82,910 Galactic disk
stars from GALAH+ DR3. We fit the median trends of low-Ia and high-Ia
populations with a two-process model, which describes stellar abundances in
terms of a prompt core-collapse and delayed Type-Ia supernova component. For
each sample star, we fit the amplitudes of these two components and compute the
residual $\Delta$[X/H] abundances from this two-parameter fit. We find RMS
residuals $\lesssim 0.07$ dex for well-measured elements and correlated
residuals among some elements (such as Ba, Y, and Zn) that indicate common
enrichment sources. From a detailed investigation of stars with large
residuals, we infer that roughly $40\%$ of the large deviations are physical
and $60\%$ are caused by problematic data such as unflagged binarity, poor
wavelength solutions, and poor telluric subtraction. As one example of a
population with distinctive abundance patterns, we identify 15 stars that have
0.3-0.6 dex enhancements of Na but normal abundances of other elements from O
to Ni and positive average residuals of Cu, Zn, Y, and Ba. We measure the
median elemental residuals of 14 open clusters, finding systematic
$\sim0.1-0.4$ dex enhancements of O, Ca, K, Y, and Ba and $\sim0.2$ dex
depletion of Cu in young clusters. Finally, we present a restricted
three-process model where we add an asymptotic giant branch star (AGB)
component to better fit Ba and Y. With the addition of the third process, we
identify a population of stars, preferentially young, that have much higher AGB
enrichment than expected from their SNIa enrichment.
