A statistical analysis of circumstellar material in Type Ia supernovae
  A key tracer of the elusive progenitor systems of Type Ia supernovae (SNe Ia)
is the detection of narrow blueshifted time-varying Na I D absorption lines,
interpreted as evidence of circumstellar material (CSM) surrounding the
progenitor system. The origin of this material is controversial, but the
simplest explanation is that it results from previous mass loss in a system
containing a white dwarf and a non-degenerate companion star. We present new
single-epoch intermediate-resolution spectra of 17 low-redshift SNe Ia taken
with XShooter on the ESO Very Large Telescope. Combining this sample with
events from the literature, we confirm an excess (~20 per cent) of SNe Ia
displaying blueshifted narrow Na I D absorption features compared to
non-blueshifted Na I D features. The host galaxies of SNe Ia displaying
blueshifted absorption profiles are skewed towards later-type galaxies,
compared to SNe Ia that show no Na I D absorption, and SNe Ia displaying
blueshifted narrow Na I D absorption features have broader light curves. The
strength of the Na I D absorption is stronger in SNe Ia displaying blueshifted
Na I D absorption features than those without blueshifted features, and the
strength of the blueshifted Na I D is correlated with the B-V colour of the SN
at maximum light. This strongly suggests the absorbing material is local to the
SN. In the context of the progenitor systems of SNe Ia, we discuss the
significance of these findings and other recent observational evidence on the
nature of SN Ia progenitors. We present a summary that suggests there are at
least two distinct populations of normal, cosmologically useful SNe Ia.
