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 supernovae observed prior to B–band maximum (Bmax). These HVF start at more than 25, 000 km s in the days after first light, and slow to about 18, 000 km s near Bmax. To recreate the Ca II near–infrared triplet (CaNIR) HVF in SN 2011fe, we consider the interaction between a Type Ia supernova and a compact circumstellar shell, employing a hydrodynamic 1–D simulation using FLASH. We generate synthetic spectra from the hydrodynamic results using syn++. We show that the CaNIR HVF and its velocity evolution is better explained by a supernova model interacting with a shell than a model without a shell, and briefly discuss the implications for progenitor models. Subject headings: supernovae: general — supernovae: individual (SN 2011fe) — line: formation — line: profiles