A Wolf-Rayet-like progenitor of supernova SN 2013cu from spectral observations of a wind

TL;DR

This paper reports the detection of Wolf-Rayet-like spectral signatures in early observations of supernova SN 2013cu, providing direct evidence of a Wolf-Rayet progenitor with increased mass loss shortly before explosion.

Contribution

It presents the first spectral detection of a Wolf-Rayet-like wind in a supernova progenitor, linking spectral features to progenitor properties and mass-loss behavior.

Findings

Detected strong emission lines indicating a Wolf-Rayet-like wind

Progenitor likely of the WN(h) subclass

Dense wind suggests increased mass loss before explosion

Abstract

The explosive fate of massive stripped Wolf-Rayet (W-R) stars is a key open question in stellar physics. An appealing option is that hydrogen-deficient W-R stars are the progenitors of some H-poor supernova (SN) explosions of Types IIb, Ib, and Ic. A blue object, having luminosity and colors consistent with those of some W-R stars, has been recently identified at the location of a SN~Ib in pre-explosion images but has not yet been conclusively determined to have been the progenitor. Similar previous works have so far only resulted in nondetections. Comparison of early photometric observations of Type Ic supernovae with theoretical models suggests that the progenitor stars had radii <10^12 cm, as expected for some W-R stars. However, the hallmark signature of W-R stars, their emission-line spectra, cannot be probed by such studies. Here, we report the detection of strong emission lines in an early-time spectrum of SN 2013cu (iPTF13ast; Type IIb) obtained ~15.5 hr after explosion ("flash spectroscopy"). We identify W-R-like wind signatures suggesting a progenitor of the WN(h) subclass. The extent of this dense wind may indicate increased mass loss from the progenitor shortly prior to its explosion, consistent with recent theoretical predictions.