  One of the main questions concerning Type Ia supernovae is the nature of the
binary companion of the exploding white dwarf. A major discriminant between
different suggested models is the presence and physical properties of
circumstellar material at the time of explosion. If present, this material will
be ionized by the ultra-violet radiation of the explosion and later recombine.
This ionization-recombination should manifest itself as time-variable
absorption features that can be detected via multi-epoch
high-spectral-resolution observations. Previous studies have shown that the
strongest effect is seen in the neutral sodium D lines. We report on
observations of neutral sodium absorption features observed in multi-epoch
high-resolution spectra of 14 Type Ia supernova events. This is the first
multi-epoch high-resolution study to include multiple SNe. No variability in
line strength that can be associated with circumstellar material is detected.
We find that ~18% of the events in the extended sample exhibit time-variable
sodium features associated with circumstellar material. We explore the
implication of this study on our understanding of the progenitor systems of
Type Ia supernovae via the current Type Ia supernova multi-epoch
high-spectral-resolution sample.
