Abundance ratios in GALAH DR2 and their implications for nucleosynthesis
  Using a sample of 70 924 stars from the second data release of the GALAH
optical spectroscopic survey, we construct median sequences of [X/Mg] vs.
[Mg/H] for 21 elements, separating the high-$\alpha$/``low-Ia'' and
low-$\alpha$/``high-Ia'' stellar populations through cuts in [Mg/Fe]. Previous
work with the near-IR APOGEE survey has shown that such sequences are nearly
independent of location in the Galactic disk, implying that they are determined
by stellar nucleosynthesis yields with little sensitivity to other chemical
evolution aspects. The separation between the two [X/Mg] sequences indicates
the relative importance of prompt and delayed enrichment mechanisms, while the
sequences' slopes indicate metallicity dependence of the yields. GALAH and
APOGEE measurements agree for some of their common elements, but differ in
sequence separation or metallicity trends for others. GALAH offers access to
nine new elements. We infer that about $75\%$ of solar C comes from core
collapse supernovae and $25\%$ from delayed mechanisms. We find core collapse
fractions of $60-80\%$ for the Fe-peak elements Sc, Ti, Cu, and Zn, with strong
metallicity dependence of the core collapse Cu yield. For the neutron capture
elements Y, Ba, and La, we infer large delayed contributions with non-monotonic
metallicity dependence. The separation of the [Eu/Mg] sequences implies that at
least $\sim30\%$ of Eu enrichment is delayed with respect to star formation. We
compare our results to predictions of several supernova and AGB yield models;
C, Na, K, Mn, and Ca all show discrepancies with models that could make them
useful diagnostics of nucleosynthesis physics.
