The Evolution of Carbon and Oxygen in the Bulge and Disk of the Milky Way

TL;DR

This paper reviews the evolution of carbon and oxygen abundances in the Milky Way's bulge and disk, comparing observations with models that incorporate metallicity-dependent stellar winds, revealing their crucial role in chemical evolution.

Contribution

It demonstrates that metallicity-dependent stellar winds in massive stars are essential to explain observed C/O ratios in the Galaxy's bulge and disk.

Findings

C/O ratio increases with metallicity in bulge and thick disk.

Models with metallicity-dependent yields match observations, unlike those without.

Steep decline of [O/Fe] ratio explained by stellar winds.

Abstract

The evolution of C and O abundances in the Milky Way can impose strong constraints on stellar nucleosynthesis and help understanding the formation and evolution of our Galaxy. The aim is to review the measured C and O abundances in the disk and bulge of the Galaxy and compare them with model predictions. We adopt two successful chemical evolution models for the bulge and the disk, which assume the same nucleosynthesis prescriptions but different histories of star formation. The data show a clear distinction between the trend of [C/O] in the thick and thin Galactic disks, while the thick disk and bulge trends are indistinguishable with a large (>0.5 dex) increase in the C/O ratio in the range from -0.1 to +0.4 dex for [O/H]. In our models we consider yields from massive stars with and without the inclusion of metallicity-dependent stellar winds. The observed increase in the [C/O] ratio with metallicity in the bulge and thick disk lies between the predictions utilizing the mass-loss rates of Maeder (1992) and those of Meynet & Maeder (2002). A model without metallicity-dependent yields completely fails to match the observations. Thus, the relative increase in carbon abundance at high metallicity appears to be due to metallicity-dependent stellar winds in massive stars. These results also explain the steep decline of the [O/Fe] ratio with [Fe/H] in the Galactic bulge, while the [Mg/Fe] ratio is enhanced at all [Fe/H]. (abridged)