A Compact Circumstellar Shell as the Source of High--velocity Features in SN 2011fe

Abstract: 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.