Assigned ID: 197 ----- Attempting Submission 1 (Thu Feb 25 22:35:07 GMT 2010) ----- HST Phase I Proposal 197 (rc_cycle18.apt) successfully submitted. ----- Attempting Submission 2 (Fri Feb 26 20:13:17 GMT 2010) ----- The core helium burning stars of the red clump (RC) are a conspicuous feature in the color-magnitude diagram of many stellar populations. Its ease of identification, along with its relative brightness (M_I ~ 0) make the RC a popular feature for HST studies of stellar populations in galaxies out to a few Mpc. Such studies generally interpret the data through comparison to theoretical isochrones. For accurate results, the theoretical predictions must be calibrated to match the RC properties of observed populations of known age and metallicity. However, no large scale studies of the luminosity of the RC currently exist in the optical bands. We propose to remedy this situation with an archival study of RC properties in star clusters in the Milky Way, LMC, and SMC. We will focus on HST images of globular clusters, but we will augment the sample with ground-based open cluster observations to extend the coverage of parameter space. The goal is to build a large and homogeneous database, through new analysis and incorporation of literature data, of cluster ages, abundances, distances, and RC photometry. This database will allow us to explore the variations in the RC luminosity as a function of age and [Fe/H] over the full range of parameter space where the RC exists, for both the V and I bands. The results will provide a fundamental calibration for all future HST studies of stellar populations and distances of nearby galaxies using the RC. They will also allow for verification or improvement of theoretical models for red giant phase evolution. This in turn will help many subjects, from stellar modeling to population synthesis and fitting of spectral energy distributions of distant galaxies.

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