Evidence for asymmetric distribution of circumstellar material around
  Type Ia Supernovae
  We study the properties of low-velocity material in the line of sight towards
nearby Type Ia Supernovae (SNe Ia) that have measured late phase nebular
velocity shifts (v_neb), thought to be an environment-independent observable.
We have found that the distribution of equivalent widths of narrow blended Na I
D1 & D2 and Ca II H & K absorption lines differs significantly between those
SNe Ia with negative and positive v_neb, with generally stronger absorption for
SNe Ia with v_neb > 0. A similar result had been found previously for the
distribution of colors of SNe Ia, which was interpreted as a dependence of the
temperature of the ejecta with viewing angle. Our work suggests that: 1) a
significant part of these differences in color should be attributed to
extinction, 2) this extinction is caused by an asymmetric distribution of
circumstellar material (CSM) and 3) the CSM absorption is generally stronger on
the side of the ejecta opposite to where the ignition occurs.
  Since it is difficult to explain 3) via any known physical processes that
occur before explosion, we argue that the asymmetry of the CSM is originated
after explosion by a stronger ionizing flux on the side of the ejecta where
ignition occurs, probably due to a stronger shock breakout and/or more exposed
radioactive material on one side of the ejecta. This result has important
implications for both progenitor and explosion models.
