2014MNRAS.439.1807B

Observations spanning a large wavelength 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 Hei 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 main-sequence mass of 12-15M_☉, that ejected a mass of 56Ni of about 0.04M_☉, with an energy of E = 8.5x10⁵⁰ ERG. Such a low-mass progenitor scenario is in full agreement with the modelling of the nebular spectrum taken at ∼ 215d 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.