Inflow, Outflow, Yields, and Stellar Population Mixing in Chemical
  Evolution Models
  Chemical evolution models are powerful tools for interpreting stellar
abundance surveys and understanding galaxy evolution. However, their
predictions depend heavily on the treatment of inflow, outflow, star formation
efficiency (SFE), the stellar initial mass function, the Type Ia supernova
delay time distribution, stellar yields, and stellar population mixing. Using
flexCE, a flexible one-zone chemical evolution code, we investigate the effects
of and trade-offs between parameters. Two critical parameters are SFE and the
outflow mass-loading parameter, which shift the knee in [O/Fe]-[Fe/H] and the
equilibrium abundances that the simulations asymptotically approach,
respectively. One-zone models with simple star formation histories follow
narrow tracks in [O/Fe]-[Fe/H] unlike the observed bimodality (separate
high-alpha and low-alpha sequences) in this plane. A mix of one-zone models
with inflow timescale and outflow mass-loading parameter variations, motivated
by the inside-out galaxy formation scenario with radial mixing, reproduces the
two sequences better than a one-zone model with two infall epochs. We present
[X/Fe]-[Fe/H] tracks for 20 elements assuming three different supernova yield
models and find some significant discrepancies with solar neighborhood
observations, especially for elements with strongly metallicity-dependent
yields. We apply principal component abundance analysis (PCAA) to the
simulations and existing data to reveal the main correlations amongst
abundances and quantify their contributions to variation in abundance space.
For the stellar population mixing scenario, the abundances of alpha-elements
and elements with metallicity-dependent yields dominate the first and second
principal components, respectively, and collectively explain 99% of the
variance in the model. flexCE is a python package available at
https://github.com/bretthandrews/flexCE.
