The Ratio of Blue to Red Supergiants in Sextans A from HST Imaging

Abstract

We have examined the ratio of blue to red (B/R) supergiants 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 He-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 B/R supergiant ratio provides an observational constraint on the relative lifetimes of these two phases which 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 ratio is a fairly strong function of mass for a single age stellar population, both changes in recent star formation rate and choice of luminosity cut-off 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 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 ratio is lower than the model by a factor of two. This result suggests that stellar rotation is an important effect in the evolution of these stars.