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We use medium-band near-infrared (NIR) Hubble Space Telescope WFC3 photometrywith model NIR spectra of Asymptotic Giant Branch (AGB) stars to develop a newtool for efficiently distinguishing carbon-rich (C-type) AGB stars fromoxygen-rich (M-type) AGB stars in galaxies at the edge of and outside the LocalGroup. We present the results of a test of this method on a region of the innerdisk of M31, where we find a surprising lack of C stars, contrary to thefindings of previous C star searches in other regions of M31. We find only 1candidate 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/Mestimates in M31. The low C/M ratio is likely due to the high metallicity inthis region which impedes stars from achieving C/O > 1 in their atmospheres.These observations provide stringent constraints evolutionary models ofmetal-rich AGB stars and suggest that there is a metallicity threshold abovewhich M stars are unable to make the transition to C stars, dramaticallyaffecting AGB mass loss and dust production and, consequently, the observedglobal properties of metal-rich galaxies.

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