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

The progenitors of Type-IIb supernovae (SNe IIb) are believed to have lost their H-rich envelopes almost completely in the direct pre-SN phase. Recently the first ‘flash spectrum’ of an SN IIb (SN 2013cu) has been presented, taken early enough to study its immediate circumstellar medium (CSM). Similar to a previous study by Groh, we analyse the structure and chemical composition of the optically thick CSM using non-local thermodynamic equilibrium(non-LTE) model atmospheres. For the first time, we take light-travel time 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 ∼0.3 M⊙ 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 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, or Yellow Hypergiant. 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.