Decoding Chemical Evolution and Nucleosynthesis

I will discuss insights from analytic and numerical models of galactic chemical evolution and observations of Milky Way elemental abundances from the SDSS APOGEE survey. Under generic model assumptions, abundances and abundance ratios approach an equilibrium in which element production from nucleosynthesis is balanced by element depletion from star formation and outflows. The efficiency of outflows required to reproduce observed abundances is strongly degenerate with the uncertain overall scale of IMF-averaged supernova yields. APOGEE observations show that the distributions of stars in (alpha,iron,age)-space change steadily across the Milky Way disk, with features that suggest radial migration of stars and complexity in the Galactic star formation history. However, the median trends of abundance ratios are remarkably stable throughout the disk and bulge, which allows these trends to provide empirical constraints on supernova nucleosynthesis that are insensitive to uncertainties in other aspects of chemical evolution. Constraining the absolute scale of IMF-averaged yields is challenging; the high observed deuterium abundance in the local ISM indirectly suggests a high yield scale, implying that most massive stars explode rather than collapse to black holes.