Program: GN-2011A-Q-16

A major obstacle in the use of Type Ia supernovae (SNe Ia) as standard candles has been discovered this year: even *after correction* for lightcurve shape and color, SNe Ia in high and low-mass galaxies have absolute magnitudes differing by 0.08 +/- 0.02 mag. The reason remains mysterious, exacerbated by the fact that we do not understand the progenitors of SNe Ia, but the galaxy mass-metallicity relation fingers the metallicity of the exploding white dwarf as the prime suspect. Unfortunately, the metallicity of the progenitor is best probed by studying early time UV features of the supernova, and their evolution to maximum light, which has been, until now, impossible. However, we have recently been granted an unprecedented Cycle 18 HST STIS ToO program (the number of ToOs allowed per year was increased in light of our proposal) to perform the pioneering study of early UV behavior. This will probe not only the SN metallicity, but the density structures in the outer layers, expected to discriminate between progenitor models. With queue scheduling of the ever-present GMOS, Gemini is essential to the success of this effort, providing the necessary time-critical screening and simultaneous ground-based optical spectra.