The Chemical Abundance Structure of the Inner Milky Way: A Signature of "Upside-Down" Disk Formation?

Understand

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}.