The Chemical Abundance Structure of the Inner Milky Way: A Signature of “Upside-down” Disk Formation

Abstract

Abstract We present a model for the distribution of stars in the inner Galaxy, , measured as a function of vertical distance from the midplane by Hayden et al. (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 to over a timescale t c , remaining constant thereafter. We assume a linear-exponential star formation history, . With a star formation efficiency timescale , an outflow mass-loading factor , , and , the model reproduces the observed locus of inner disk stars in and the metallicity distribution functions (MDFs) measured by H15 at , , and . Substantial changes to model parameters lead to disagreement with the H15 data; for example, models with or fail to match the observed MDF at high- and low- , respectively. The inferred scale-height evolution, with z h (t) dropping on a timescale 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 found by H15. Our scenario for the evolution of the inner disk can be tested by future measurements of the -distribution and the age–metallicity distribution at .