Light-travel-time diagnostics in early Supernova spectra: substantial mass loss of the IIb progenitor of SN 2013cu through a superwind

TL;DR

This study analyzes early spectra of SN 2013cu, revealing a substantial, previously underestimated circumstellar mass loss through a superwind, highlighting the importance of light-travel time effects in supernova diagnostics.

Contribution

It introduces the first model accounting for light-travel time effects in supernova spectra, leading to revised estimates of circumstellar mass and mass-loss rates for SN 2013cu.

Findings

CSM mass lower limit ~0.3 solar masses

Mass-loss rate exceeds expectations by 100 times

Progenitor likely in late RSG or LBV stage

Abstract

The progenitors of type-IIb supernovae (SNe) are believed to have lost their H-rich envelopes almost completely in the direct pre-SN phase. Recently the first 'flash spectrum' of a SN IIb (SN2013cu) has been presented, taken early enough to study its immediate circumstellar medium (CSM). Similar to a previous study by Groh (2014) we analyse the structure and chemical composition of the optically-thick CSM using non-LTE model atmospheres. For the first time we take light-travel time (LTT) effects on the spectrum formation into account, which affect the shapes and strengths of the observable emission lines, as well as the inferred SN luminosity. Based on the new CSM parameters we estimate a lower limit of ~0.3Msun for the CSM mass, which is a factor 10-100 higher than previous estimates. The spectral fit implies a CSM in the form of a homogeneous and spherically symmetric superwind whose mass-loss rate exceeds common expectations by up to two orders of magnitude. The derived chemical composition is in agreement with a progenitor that has just left, or is just about to leave the Red-Supergiant (RSG) stage, confirming the standard picture for the origin of SNe IIb. Due to its extreme mass loss the SN progenitor will likely appear as extreme RSG, Luminous Blue Variable (LBV), or Yellow Hypergiant (YHG). The direct detection of a superwind, and the high inferred CSM mass suggest that stellar wind mass loss may play an important role in the formation of SNe IIb.