Observational Constraints on Models of Rapidly Evolving Luminous Stars

Abstract

Resolved stellar populations in galaxies are excellent laboratories for testing our understanding of galaxy formation, integrated colors and luminosities, supernova progenitor masses, and energy input from stellar feedback. However, the usefulness of resolved stellar populations rests on the ability to accurately model the evolution of the underlying stars. In this dissertation, I present three projects, the first uses stellar evolution models of hot post-horizontal branch stars to explain the some of the of excess ultra-violet (UV) flux emitted from the center of the Andromeda galaxy. This flux was imaged as part of the Panchromatic Hubble Andromeda Treasury HST multi-cycle program, and excess of UV light from the centers of large elliptical galaxies has been well documented. Therefore, the results of this project go beyond stellar evolution and understanding M31. I find that this population of stars are indeed failed AGB stars or left the AGB early. I also find the stars producing the excess UV light are strongly concentrated in the central regions of the M31 bulge, which must shed light on the progenitor stars. The subsequent two projects each constrain uncertain aspects of uncertain phases of stellar evolution using nearby dwarf galaxies from the HST ACS Nearby galaxy Survey Treasury and a follow-up HST/SNAP campaign which imaged a subset of ANGST galaxies in the near infrared. The two uncertain phases of stellar evolution studied are the luminous core Helium burning (HeB) phase and the thermally pulsating AGB phase. Dwarf galaxies from the ANGST are an ideal environment to test new models because they span ~2 dex in metallicity, more than 30 galaxies have significant HeB populations, and we have HST/NIR follow up data for 26 galaxies with a large AGB population. I find that the convection occurring the HeB stars must increase in strength with increasing mass, and I find constrains on the mass loss prescriptions for low metallicity TP-AGB stars from 0.8- ~4 Msun}