Ultraviolet Spectroscopy of Extreme Standard Candles

Despite their widespread use as standard candles to measure cosmological distances, Type Ia supernovae are not a homogeneous group of explosions. There are variations in optical light curve shape and ejecta velocity, for example. The sensitivity of the UV to differences in progenitor systems, explosion physics, and environmental conditions makes it a vital regime to determine the physical cause and magnitude of the differences and their effect on the optical bands currently used for cosmological measurements. The goal of understanding the range of progenitor and explosion properties of SNe Ia can be advanced by obtaining UV spectra covering the range of Type Ia supernova properties. The proposed observations will sparsely sample the multi-dimensional parameter spaces of the observed and physical differences between various classes. The goal is to bring order to these disparate objects by determining which are actually physically related and by what parameters. Combined with previous HST observations, these observations will form the definitive observational sample for building theoretical models, constraining progenitor environments, and comparing with high-redshift supernovae from past and future observatories including LSST and WFIRST.