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

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 H I 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 3-4 M He-core progenitor star, corresponding to a mainsequence mass of 12-15 M , that ejected a mass of 56 Ni of about 0.04 M , with an energy of E = 8.5 × 10 50 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