Weak CN and Carbon Stars in M31 and M33: I. Spectroscopic and Kinematical Properties

Our large neighboring galaxy Andromeda (M31) and its dwarf spiral companion Triangulum (M33) are excellent laboratories for stellar evolution studies, as each galaxy’s vast collection of stars is located at essentially the same distance from us. M31 is more chemically enriched on average (higher mean metallicity) than our Milky Way (MW), while M33 is less chemically enriched than the MW. Of the three galaxies, M33 has the highest star formation rate, followed by M31 and the MW. As such, M31 and M33 provide excellent context for MW stellar population studies in the Solar neighborhood. In this poster, the first of two companion posters, we present the discovery, spectral characteristics, and kinematical properties of stars that display an unusual weak CN (cyanogen) spectral feature at ~8000 Å alongside other normal spectral absorption features. Our study is based on analysis of Keck DEIMOS low- and medium-resolution spectra of M31 and M33’s resolved stellar population from the Spectroscopic and Photometric Survey of Andromeda’s Stellar Halo (SPLASH) survey. We present a comparative study of the spectroscopic properties of these so-called “weak CN” stars, carbon stars (whose spectra contain strong CN, C2, CH, etc. spectral absorption features), and normal cool stars (whose spectra contain absorption features that are associated with oxygen-rich molecules). We compare the spectra of all M31 and M33 stars in our sample to co-added spectral templates of weak CN and carbon stars in a COMparison to Empirical Template (COMET) plot. Our group has shown that short lived massive stars in M31’s disk have more coherent kinematics than long lived low mass stars (e.g., Dorman et al. 2015; Quirk et al. 2019). Our kinematical analysis of weak CN and carbon stars in M31 and M33 therefore places indirect constraints on the average stellar mass of each of these subgroups of rare stars.

TR conducted his research under the auspices of the Science Internship Program (SIP) at the University of California Santa Cruz; he was a fellow of the Cal Poly San Luis Obispo STEM Teacher and Researcher (STAR) program. This research was funded in part by the National Science Foundation and NASA/STScI.