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

Abstract

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.

Accepted for publication in ApJ

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• Characterisation of Galactic carbon stars and related stars from Gaia-EDR3
• The period--luminosity relation for Mira variables in the Milky Way using Gaia DR3: a further distance anchor for $H_0$
• A Dramatic Decrease in Carbon Star Formation in M31
• Carbon stars as standard candles: II. The median J magnitude as a distance indicator
• The Triangulum Extended (TREX) Survey: The Stellar Disk Dynamics of M33 as a Function of Stellar Age
• Carbon Stars in the Satellites and Halo of M31
• A Census of Thermally-Pulsing AGB stars in the Andromeda Galaxy and a First Estimate of their Contribution to the Global Dust Budget
• Emission-line stars in M31 from the SPLASH and PHAT surveys
• The frequency by mass of Galactic carbon stars inferred from Gaia measurements of star cluster members