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We explore the ratio (C/M) of carbon-rich to oxygen-rich thermally pulsingasymptotic giant branch(TP-AGB) stars in the disk of M31 using a combination ofmoderate-resolution optical spectroscopy from the Spectroscopic Landscape ofAndromeda's Stellar Halo (SPLASH) survey and six-filter Hubble Space Telescopephotometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey.Carbonstars were identified spectroscopically. Oxygen-rich M-stars were identifedusing three different photometric definitions designed to mimic, and thusevaluate, selection techniques common in the literature. We calculate the C/Mratio as a function of galactocentric radius, present-day gas-phase oxygenabundance, stellar metallicity, age (via proxy defined as the ratio of TP-AGBstars to red giant branch, RGB, stars), and mean star formation rate over thelast 400 Myr. We find statistically significant correlations between log(C/M)and all parameters. These trends are consistent across different M-starselection methods, though the fiducial values change. Of particular note is ourobserved relationship between log(C/M) and stellar metallicity, which is fullyconsistent with the trend seen across Local Group satellite galaxies. The factthat this trend persists in stellar populations with very different starformation histories indicates that the C/M ratio is governed by stellarproperties alone.

A Spectroscopic and Photometric Exploration of the C/M Ratio in the Disk of M31