SN2011hs: 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 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 Msun He-core progenitor star,
corresponding to a main sequence mass of 12-15 Msun, that ejected a mass of
56Ni of about 0.04 Msun, with an energy of E= 8.5 x 10^50 erg. Such a low-mass
progenitor scenario is in full agreement with the modelling of the nebular
spectrum taken at $\sim$215 days from maximum. From the modelling of the
adiabatic cooling phase, we infer a progenitor radius of $\approx$500-600 Rsun,
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.
