Using Difference Imaging to Study Faint Miras in the Panchromatic Hubble Andromeda Treasury

Variable stars have long been used as standard candles and as a testbed for models of stellar evolution. Here we focus on Miras—characteristically large, red, long-period variables. Miras have a radius hundred of times larger than the sun, and are thousands of times more luminous than the sun. Populations of long-period variable stars like Miras have been extensively cataloged and analyzed in nearby galaxies, like the LMC, SMC, and the Milky Way. Such stars are very common and can be used as distance and age indicators, which make them useful for determining the 3D structure of their host galaxies. The period, amplitude, and luminosity of Miras have been found to correlate with their metallicity and age, giving insight into star formation and evolution. We use statistical and difference-imaging methods to search for Miras using data from the Panchromatic Hubble Andromeda Treasury (PHAT) survey. The latter contains data for more than 117 million stars in the Andromeda M31) galaxy, which is the the closest spiral galaxy with similar mass and age as the Milky Way, and has its stars relatively equidistant from observers, making PHAT data easier to interpret. So far, there has not been an extensive catalog of long-period variables in M31, which, unlike the metal-poor Milky Way, has a wider age and metallicity range of stellar populations. Using photometric data, we obtained 700k Mira candidates out of PHAT’s 117 million stars, and verified a number of these stars as Miras with difference-imaging. These Miras can be followed up in the future by large-aperture telescopes to gather data for their spectroscopic and period analyses. The distributions of age, period, amplitude, and locations of Miras in M31 and the Magellanic Clouds could be compared to constrain metallicity effect in the various processes governing stellar evolution.

This research was funded in part by the NSF and NASA/STScI. SZ and PT conducted their research under the auspices of the Science Internship Program (SIP) at the UC Santa Cruz.