Electron scattering wings on lines in interacting supernovae

TL;DR

This paper models how electron scattering in ionized circumstellar media influences emission line profiles in supernovae, providing insights into the gas properties and explaining observed asymmetries.

Contribution

It introduces a detailed scattering model for supernova emission lines, accounting for optical depth and gas density profiles, and applies it to observed supernovae to interpret line asymmetries.

Findings

Line profiles are exponential at low velocities, steepening at higher velocities.

Optical depth affects the strength of line wings and the ratio of narrow to broad components.

Red asymmetries can be explained by scattering in fast wind regions.

Abstract

We consider the effect of electron scattering on lines emitted as a result of supernova interaction with a circumstellar medium, assuming that the scattering occurs in ionized gas in the preshock circumstellar medium. The single scattering case gives the broad component in the limit of low optical depth, showing a velocity full width half maximum that is close to the thermal velocities of electrons. The line shape is approximately exponential at low velocities and steepens at higher velocities. At higher optical depths, the line profile remains exponential at low velocities, but wings strengthen with increasing optical depth. In addition to the line width, the ratio of narrow to broad (scattered) line strength is a possible diagnostic of the gas. The results depend on the density profile of the circumstellar gas, especially if the scattering and photon creation occur in different regions. We apply the scattering model to a number of supernovae, including Type IIn and Type Ia-CSM events. The asymmetry to the red found in some cases can be explained by scattering in a fast wind region which is indicated by observations.