A compact circumstellar shell model as a source of high velocity features in Type Ia supernovae
High-velocity features (HVF) of Type Ia supernovae are found in calcium, silicon, iron, and other elements. These features are observed only at early epochs — generally within the first one to three weeks after the supernova explodes. These features can provide insight into the cause of the supernova or the composition of the system prior to the explosion. We consider a model in which the HVF are the result of an interaction between the supernova and compact circumstellar shell. Hydrodynamic simulations of the interaction are performed, and synthetic spectra are generated using syn++ from the results of the simulation to explore the effect of the shell and interaction. Of the masses, sizes, and density profiles of the shell that we consider, only density profiles that have a steep gradient at the outer edge may be ruled out based on the shape and evolution of the Ca II near-infrared feature. The observed features of SN 2011fe are then fit using these models, finding that the mass of the shell is not well constrained, but the velocity of the feature near peak brightness is better explained by a shell with a mass of 0.005 M ⊙ . I describe a method to estimate the calcium abundance in supernova or shell material based on the results of the fit assuming purely radiative excitation and ionization, and simultaneously solving the excitation- and ion-states for a given element. Possible compositions of the shell and ejecta are explored using tardis for spectral synthesis, considering shells that consist of a solar abundance of metals within a hydrogen, helium, or carbon and oxygen substrate, as well compositions based upon a helium envelope detonation. The material in the shell is more likely to contain a Solar or super-Solar abundance of calcium; the substrate within which this calcium lies is not detectable using spectra.