The lack of carbon stars in the Galactic bulge

TL;DR

This study investigates why the Galactic bulge has few carbon stars by modeling AGB star evolution, highlighting the potential role of high mass loss rates over oxygen overabundance in explaining their scarcity.

Contribution

It introduces a population synthesis model to analyze the impact of oxygen overabundance and mass loss rates on carbon star formation in the Galactic bulge.

Findings

Oxygen overabundance alone cannot explain the lack of carbon stars.

High mass loss rates may suppress carbon star formation.

Oxygen overabundance is about twice that of the solar neighborhood.

Abstract

In order to explain the lack of carbon stars in the Galactic bulge, we have made a detailed study of thermal pulse - asymptotic giant branch stars by using a population synthesis code. The effects of the oxygen overabundance and the mass loss rate on the ratio of the number of carbon stars to that of oxygen stars in the Galactic bulge are discussed. We find that the oxygen overabundance which is about twice as large as that in the solar neighbourhood (close to the present observations) is insufficient to explain the rareness of carbon stars in the bulge. We suggest that the large mass loss rate may serve as a controlling factor in the ratio of the number of carbon stars to that of oxygen stars.