Theoretical Clues to the Ultraviolet Diversity of Type Ia Supernovae
  The effect of metallicity on the observed light of Type Ia supernovae (SNe
Ia) could lead to systematic errors as the absolute magnitudes of local and
distant SNe Ia are compared to measure luminosity distances and determine
cosmological parameters. The UV light may be especially sensitive to
metallicity, though different modeling methods disagree as to the magnitude,
wavelength dependence, and even the sign of the effect. The outer density
structure, ^56 Ni, and to a lesser degree asphericity, also impact the UV. We
compute synthetic photometry of various metallicity-dependent models and
compare to UV/optical photometry from the Swift Ultra-Violet/Optical Telescope.
We find that the scatter in the mid-UV to near-UV colors is larger than
predicted by changes in metallicity alone and is not consistent with reddening.
We demonstrate that a recently employed method to determine relative abundances
using UV spectra can be done using UVOT photometry, but we warn that accurate
results require an accurate model of the cause of the variations. The abundance
of UV photometry now available should provide constraints on models that
typically rely on UV spectroscopy for constraining metallicity, density, and
other parameters. Nevertheless, UV spectroscopy for a variety of SN explosions
is still needed to guide the creation of accurate models. A better
understanding of the influences affecting the UV is important for using SNe Ia
as cosmological probes, as the UV light may test whether SNe Ia are
significantly affected by evolutionary effects.
