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