A Compact Circumstellar Shell as the Source of High--velocity Features
  in SN 2011fe
  High-velocity features (HVF), especially of Ca II, are frequently seen in
Type Ia supernova observed prior to B-band maximum (Bmax). These HVF evolve in
velocity from more than 25,000 km/s, in the days after first light, to about
18,000 km/s near Bmax. To recreate the evolution of the Ca II near-infrared
triplet (CaNIR) HVF in SN 2011fe, we consider the interaction between a model
Type Ia supernova and compact circumstellar shells with masses between 0.003
solar masses and 0.012 solar masses. We fit the observed CaNIR feature using
synthetic spectra generated from the models using syn++. The CaNIR feature is
better explained by the supernova model interacting with a shell than the model
without a shell, with a shell of mass 0.005 solar masses tending to be better
fitting than the other shells. The evolution of the optical depth of CaNIR
suggests that the ionization state of calcium within the ejecta and shell is
not constant. We discuss the method used to measure the observed velocity of
CaNIR and other features and conclude that HVF or other components can be
falsely identified. We briefly discuss the possible origin of the shells and
the implications for the progenitor system of the supernova.
