First day of type IIP supernova SN 2013fs: H$\alpha$ from preshock accelerated gas

TL;DR

This paper investigates the asymmetric Hα emission in SN 2013fs, attributing it to preshock gas acceleration and optical effects, providing a model that matches observed spectra shortly after shock breakout.

Contribution

It introduces a spherical model accounting for preshock gas acceleration to explain Hα asymmetry in supernova spectra, a novel approach for early supernova observations.

Findings

High velocity preshock gas (~3000 km/s) causes asymmetry.

Low Hα Sobolev optical depth influences emission profile.

Occultation by the photosphere affects observed asymmetry.

Abstract

I explore the origin of an asymmetry of the H$\alpha$ emission from a circumstellar (CS) shell around type IIP supernova SN 2013fs in the spectrum taken 10.3\,h after the shock breakout. A spherical model of the H$\alpha$ emission from the CS shell that takes into account a preshock gas acceleration by the supernova radiation permits us to successfully reproduce the \ha H$\alpha$ profile. Principal factors responsible for the H$\alpha$ asymmetry are the high velocity of the accelerated CS preshock gas ($\sim 3000$\,km s$^{-1}$) and a low H$\alpha$ Sobolev optical depth in a combination with an occultation of the H$\alpha$ emission by the photosphere.