On the Source of the Dust Extinction in Type Ia Supernovae and the
  Discovery of Anomalously Strong Na I Absorption
  High-dispersion observations of the Na I D 5890, 5896 and K I 7665, 7699
interstellar lines, and the diffuse interstellar band at 5780 Angstroms in the
spectra of 32 Type Ia supernovae are used as an independent means of probing
dust extinction. We show that the dust extinction of the objects where the
diffuse interstellar band at 5780 Angstroms is detected is consistent with the
visual extinction derived from the supernova colors. This strongly suggests
that the dust producing the extinction is predominantly located in the
interstellar medium of the host galaxies and not in circumstellar material
associated with the progenitor system. One quarter of the supernovae display
anomalously large Na I column densities in comparison to the amount of dust
extinction derived from their colors. Remarkably, all of the cases of unusually
strong Na I D absorption correspond to "Blueshifted" profiles in the
classification scheme of Sternberg et al. (2011). This coincidence suggests
that outflowing circumstellar gas is responsible for at least some of the cases
of anomalously large Na I column densities. Two supernovae with unusually
strong Na I D absorption showed essentially normal K I column densities for the
dust extinction implied by their colors, but this does not appear to be a
universal characteristic. Overall, we find the most accurate predictor of
individual supernova extinction to be the equivalent width of the diffuse
interstellar band at 5780 Angstroms, and provide an empirical relation for its
use. Finally, we identify ways of producing significant enhancements of the Na
abundance of circumstellar material in both the single-degenerate and
double-degenerate scenarios for the progenitor system.
