Weak CN and Carbon Stars in M31 and M33: II. Photometric Properties

The Andromeda spiral galaxy (M31), its companion the Triangulum dwarf spiral galaxy (M33), and our Milky Way galaxy are the three largest members of the Local Group. The combination of M31 and M33’s proximity to us and our external viewpoint make them ideal targets for the study of resolved stellar populations. In this poster, the second of two companion posters, we present a comparison between the recently discovered “weak CN” and carbon star populations in the disks of M31 and M33 in color-magnitude diagram (CMD) space. These galaxies were imaged with the Hubble Space Telescope’s Advanced Camera for Surveys and Wide Field Camera 3 imagers as part of the Panchromatic Hubble Andromeda Treasury (PHAT) survey and its M33 extension. Our CMD analysis was based on five of the six PHAT filters: F336W in the ultraviolet, F475W and F814W in the optical, and F110W and F160W in the near infrared. The location of weak CN and carbon stars in these CMDs is compared to theoretical stellar tracks and isochrones from the PARSEC library. Weak CN stars lie on the near-vertical He-burning sequence associated with evolved stars in the mass range 4-10 Msun while carbon stars are associated with intermediate mass (2-3 Msun) asymptotic giant branch stars and lie above the tip of the old low mass red giant branch. Weak CN stars are more luminous and have higher effective temperatures (bluer colors) on average than carbon stars. The weak CN sequence is more pronounced in M33 than in M31 because: (1) M33 has a high and near constant star formation rate in contrast to M31’s relatively low declining star formation rate, as a result of which M33 contains a higher fraction of massive stars at present; (2) M33 has a lower mean metallicity and spread than M31 so the He-burning sequence has a smaller color spread; and (3) M33’s disk is more face-on than M31’s so there is less smearing by differential reddening by dust. SSC, TR, AGA, JD, and OG conducted their research under the auspices of the Science Internship Program (SIP) at the University of California Santa Cruz; TR 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.