Assigned ID: 68 ----- Attempting Submission 1 (Tue Feb 22 04:47:38 GMT 2011) ----- HST Phase I Proposal 68 (BRratio_AR.c19.v7.submitted.pdf.apt) successfully submitted. ----- Attempting Submission 2 (Tue Feb 22 04:49:43 GMT 2011) ----- HST Phase I Proposal 68 (BRratio_AR.c19.v7.submitted.pdf.apt) successfully submitted. ----- Attempting Submission 3 (Fri Feb 25 04:27:54 GMT 2011) ----- The lives and deaths of massive stars are intricately linked to the evolution of galaxies. Yet, despite their integral importance to understanding galaxy evolution, models of massive stars are inconsistent with observations. These uncertainties can be traced to limited observational constraints available for improving massive star models. A sensitive test of the underlying physics of massive stars, e.g., convection, rotation, and mass loss is to measure the ratio of blue core helium burning stars (BHeB) to red core helium burning stars (RHeB), 5-20Msun stars in the stage evolution immediately following the main sequence. Even the most sophisticated models cannot accurately predict the observed ratio over a range of metallicities, suggesting an insufficient understanding of the underlying physics. However, observational measurements of this ratio over a wide range of environments would provide substantial constraints on the physical parameters governing the evolution of all stars >5 Msun. We propose to place stringent observational constraints on the physics of massive star evolution by uniformly measuring the B/R HeB ratio in a wide range of galaxies. The HST archive contains high quality optical imaging of resolved stellar populations of dozens of nearby galaxies. From the ANGST program, we identified 38 galaxies, spanning ~2 dex in metallicity that have significant BHeB and RHeB populations. Using this sample, we will empirically characterize the colors of the BHeB and RHeB sequences as a function of luminosity and metallicity, measure the B/R ratio, and constrain the lifetimes of the BHeB and RHeBs in the Padova stellar evolution models and the Cambridge STARS code.

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