The Chemical Abundance Structure of the Inner Milky Way: A Signature of
  "Upside-Down" Disk Formation?
  We present a model for the [alpha/Fe]-[Fe/H] distribution of stars in the
inner Galaxy, R=3-5 kpc, measured as a function of vertical distance |z| from
the midplane by Hayden et al. (2015, H15). Motivated by an "upside-down"
scenario for thick disk formation, in which the thickness of the star-forming
gas layer contracts as the stellar mass of the disk grows, we combine one-zone
chemical evolution with a simple prescription in which the scale-height of the
stellar distribution drops linearly from z_h=0.8 kpc to z_h=0.2 kpc over a
timescale t_c, remaining constant thereafter. We assume a linear-exponential
star-formation history, SFR ~ te^{-t/t_sf}. With a star-formation efficiency
timescale of 2 Gyr, an outflow mass-loading factor of 1.5, t_sf=3 Gyr, and
t_c=2.5 Gyr, the model reproduces the observed locus of inner disk stars in
[alpha/Fe]-[Fe/H] and the metallicity distribution functions (MDFs) measured by
H15 at |z|=0-0.5 kpc, 0.5-1 kpc, and 1-2 kpc. Substantial changes to model
parameters lead to disagreement with the H15 data; for example, models with
t_c=1 Gyr or t_sf=1 Gyr fail to match the observed MDF at high-|z| and low-|z|,
respectively. The inferred scale-height evolution, with z_h(t) dropping on a
timescale t_c ~ t_sf at large lookback times, favors upside-down formation over
dynamical heating of an initially thin stellar population as the primary
mechanism regulating disk thickness. The failure of our short-t_c models
suggests that any model in which thick disk formation is a discrete event will
not reproduce the continuous dependence of the MDF on |z| found by H15. Our
scenario for the evolution of the inner disk can be tested by future
measurements of the |z|-distribution and the age-metallicity distribution at
R=3-5 kpc.
