SN 2011hs: a fast and faint type IIb supernova from a supergiant progenitor

Observations spanning a largewavelength range, from X-ray to radio, of the Type IIb supernova (SN) 2011hs are presented, covering its evolution during the first year after explosion. The optical light curve presents a narrower shape and a fainter luminosity at peak than previously observed for Type IIb SNe. High expansion velocities are measured from the broad absorption HI and He I lines. From the comparison of the bolometric light curve and the time evolution of the photospheric velocities with hydrodynamical models, we found that SN 2011hs is consistent with the explosion of a 3-4M⊙ He-core progenitor star, corresponding to a mainsequence mass of 12-15M⊙, that ejected a mass of ⁵⁶Ni of about 0.04M⊙, with an energy of E = 8.5 × 10⁵⁰ erg. Such a low-mass progenitor scenario is in full agreement with the modelling of the nebular spectrum taken at ~215 d from maximum. From the modelling of the adiabatic cooling phase, we infer a progenitor radius of ̃500-600 R⊙, clearly pointing to an extended progenitor star. The radio light curve of SN 2011hs yields a peak luminosity similar to that of SN 1993J, but with a higher mass-loss rate and a wind density possibly more similar to that of SN 2001ig. Although no significant deviations from a smooth decline have been found in the radio light curves, we cannot rule out the presence of a binary companion star.