Studying the Diversity of Type Ia Supernovae in the Ultraviolet:
  Comparing Models with Observations
  In the ultraviolet (UV), Type Ia supernovae (SNe Ia) show a much larger
diversity in their properties than in the optical. Using a stationary
Monte-Carlo radiative transfer code, a grid of spectra at maximum light was
created varying bolometric luminosity and the amount of metals in the outer
layers of the SN ejecta. This model grid is then compared to a sample of
high-redshift SNe Ia in order to test whether the observed diversities can be
explained by luminosity and metallicity changes alone. The dispersion in
broadband UV flux and colours at approximately constant optical spectrum can be
readily matched by the model grid. In particular, the UV1-b colour is found to
be a good tracer of metal content of the outer ejecta, which may in turn
reflect on the metallicity of the SN progenitor. The models are less successful
in reproducing other observed trends, such as the wavelengths of key UV
features, which are dominated by reverse fluorescence photons from the optical,
or intermediate band photometric indices. This can be explained in terms of the
greater sensitivity of these detailed observables to modest changes in the
relative abundances. Specifically, no single element is responsible for the
observed trends. Due to their complex origin, these trends do not appear to be
good indicators of either luminosity or metallicity.
