Old Metal-Rich Globulars Constrained by the Mid-Ultraviolet

The goal of our Hubble Treasury Program is to provide observational and theoretical spectral templates for interpreting the age metallicity and population of old stellar systems. To this end we are calculating stellar mid-UV and optical spectra from first principles for comparison with observed spectra. Here we present the Peterson et al. (2003) comparison of coadditions of such spectral calculations to archival mid-UV and optical spectra of the globular cluster G1 in Andromeda (M31). Our composite spectra were generated by coadding the fluxes calculated for four to six individual stellar models weighted by the model radius squared times the relative number of stars represented by the model. We attain an excellent match to the Ponder et al. (1998) FOS observations but only when the calculations include stars on the cool blue horizontal branch (BHB) and hot extreme horizontal branch (EHB). Our metallicity [Fe/H] = -0.8 and BHB fraction both agree well with the results of Rich et al. (1996) from the G1 color-magnitude diagram. Our G1 turnoff temperature is 6000K - 6100K that of the Galactic halo. We conclude that G1 is old and mildly metal-poor but nonetheless harbors both BHB and EHB stars. Only the UV can discern them.