The Ratio of Blue to Red Supergiants in Sextans A from Hubble Space Telescope Imaging

Abstract

We have examined the ratio of blue to red supergiants (B/R) in the dwarf irregular galaxy Sextans A. The supergiants were identified in previously published stellar photometry measured from Hubble Space Telescope imaging. The high-resolution imaging and low-dust environment provided high photometric accuracy such that the main sequence and blue helium-burning supergiants are clearly separated. This allows us to isolate the He-burning phase at both the red and blue ends of the so-called blue loops. The supergiant B/R provides an observational constraint on the relative lifetimes of these two phases that is a sensitive test for convection, mass-loss, and rotation parameters. These parameters have direct implications for the period-luminosity relationship for Cepheid variable stars. Previous studies have used a single number to represent this ratio. However, since the B/R is a fairly strong function of mass for a single-age stellar population, both changes in recent star formation rate and choice of luminosity cutoff can dramatically affect the result. We have analyzed the ratio as a function of age, or, equivalently, mass. This method eliminates the confusion of unknown star formation histories so that the B/R can be a more reliable diagnostic tool. We compare the result with a model based on stellar evolution tracks of an appropriate metallicity. The functional form of the observed ratio matches the model extremely well. However, the observed B/R is lower than the model by a factor of 2. This result suggests that stellar rotation is an important effect in the evolution of these stars. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.