Ultraviolet Diversity of Type Ia Supernovae
  Ultraviolet (UV) observations of Type Ia supernovae (SNe Ia) probe the
outermost layers of the explosion, and UV spectra of SNe Ia are expected to be
extremely sensitive to differences in progenitor composition and the details of
the explosion. Here we present the first study of a sample of high
signal-to-noise ratio SN Ia spectra that extend blueward of 2900 A. We focus on
spectra taken within 5 days of maximum brightness. Our sample of ten SNe Ia
spans the majority of the parameter space of SN Ia optical diversity. We find
that SNe Ia have significantly more diversity in the UV than in the optical,
with the spectral variance continuing to increase with decreasing wavelengths
until at least 1800 A (the limit of our data). The majority of the UV variance
correlates with optical light-curve shape, while there are no obvious and
unique correlations between spectral shape and either ejecta velocity or
host-galaxy morphology. Using light-curve shape as the primary variable, we
create a UV spectral model for SNe Ia at peak brightness. With the model, we
can examine how individual SNe vary relative to expectations based on only
their light-curve shape. Doing this, we confirm an excess of flux for SN 2011fe
at short wavelengths, consistent with its progenitor having a subsolar
metallicity. While most other SNe Ia do not show large deviations from the
model, ASASSN-14lp has a deficit of flux at short wavelengths, suggesting that
its progenitor was relatively metal rich.
