Chemical evolution of the Galactic bulge: different stellar populations and possible gradients

TL;DR

This study models the chemical evolution of the Galactic bulge, revealing two distinct stellar populations with different formation timescales and predicting metallicity and element abundance gradients within the bulge.

Contribution

It introduces a detailed chemical evolution model explaining the formation of two stellar populations and predicts their abundance gradients and spatial distributions.

Findings

Old, metal-poor population formed rapidly within 0.1-0.3 Gyr.

Metal-rich population formed over 3 Gyr.

Predicted metallicity and element abundance gradients within the bulge.

Abstract

We compute the chemical evolution of the Galactic bulge to explain the existence of two main stellar populations recently observed. After comparing model results and observational data we suggest that the old more metal poor stellar population formed very fast (on a timescale of 0.1-0.3 Gyr) by means of an intense burst of star formation and an initial mass function flatter than in the solar vicinity whereas the metal rich population formed on a longer timescale (3 Gyr). We predict differences in the mean abundances of the two populations (-0.52 dex for <[Fe/H]>) which can be interpreted as a metallicity gradients. We also predict possible gradients for Fe, O, Mg, Si, S and Ba between sub-populations inside the metal poor population itself (e.g. -0.145 dex for <[Fe/H]>). Finally, by means of a chemo-dynamical model following a dissipational collapse, we predict a gradient inside 500 pc from the Galactic center of -0.26 dex kpc^{-1} in Fe.