Is there a metallicity ceiling to form carbon stars? - A novel technique reveals a scarcity of C stars in the inner M31 disk

TL;DR

This study introduces a new near-infrared photometric technique to distinguish AGB star types, revealing a scarcity of carbon stars in the metal-rich inner M31 disk, indicating a metallicity threshold for C star formation.

Contribution

The paper presents a novel method for identifying C and M type AGB stars using NIR photometry, and applies it to find a significant scarcity of C stars in a high-metallicity region of M31.

Findings

Very low C/M ratio in the inner M31 disk

High metallicity inhibits C star formation

Implications for AGB evolution and galaxy properties

Abstract

We use medium-band near-infrared (NIR) Hubble Space Telescope WFC3 photometry with model NIR spectra of Asymptotic Giant Branch (AGB) stars to develop a new tool for efficiently distinguishing carbon-rich (C-type) AGB stars from oxygen-rich (M-type) AGB stars in galaxies at the edge of and outside the Local Group. We present the results of a test of this method on a region of the inner disk of M31, where we find a surprising lack of C stars, contrary to the findings of previous C star searches in other regions of M31. We find only 1 candidate C star (plus up to 6 additional, less certain C stars candidates), resulting in an extremely low ratio of C to M stars (C/M = (3.3(+20,-0.1))x10^-4) that is 1-2 orders of magnitude lower than other C/M estimates in M31. The low C/M ratio is likely due to the high metallicity in this region which impedes stars from achieving C/O > 1 in their atmospheres. These observations provide stringent constraints evolutionary models of metal-rich AGB stars and suggest that there is a metallicity threshold above which M stars are unable to make the transition to C stars, dramatically affecting AGB mass loss and dust production and, consequently, the observed global properties of metal-rich galaxies.