A Spectroscopic and Photometric Exploration of the C/M Ratio in the Disk
  of M31
  We explore the ratio (C/M) of carbon-rich to oxygen-rich thermally pulsing
asymptotic giant branch(TP-AGB) stars in the disk of M31 using a combination of
moderate-resolution optical spectroscopy from the Spectroscopic Landscape of
Andromeda's Stellar Halo (SPLASH) survey and six-filter Hubble Space Telescope
photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey.Carbon
stars were identified spectroscopically. Oxygen-rich M-stars were identifed
using three different photometric definitions designed to mimic, and thus
evaluate, selection techniques common in the literature. We calculate the C/M
ratio as a function of galactocentric radius, present-day gas-phase oxygen
abundance, stellar metallicity, age (via proxy defined as the ratio of TP-AGB
stars to red giant branch, RGB, stars), and mean star formation rate over the
last 400 Myr. We find statistically significant correlations between log(C/M)
and all parameters. These trends are consistent across different M-star
selection methods, though the fiducial values change. Of particular note is our
observed relationship between log(C/M) and stellar metallicity, which is fully
consistent with the trend seen across Local Group satellite galaxies. The fact
that this trend persists in stellar populations with very different star
formation histories indicates that the C/M ratio is governed by stellar
properties alone.
