2013MNRAS.436.3614S

We present nebular spectra of the nearby Type IIb supernova (SN) 2011dh taken between 201 and 678d after core collapse. At these late times, SN 2011dh exhibits strong emission lines including a broad and persistent Hα feature. New models of the nebular spectra confirm that the progenitor of SN 2011dh was a low-mass giant (M ~ 13-15M_☉) that ejected ∼ 0.07M_☉ of 56Ni and ∼ 0.27M_☉ of oxygen at the time of explosion, consistent with the recent disappearance of a candidate yellow supergiant progenitor. We show that light from the SN location is dominated by the fading SN at very late times ( ∼ 2yr) and not, for example, by a binary companion or a background source. We present evidence for interaction between the expanding SN blast wave and a circumstellar medium at late times and show that the SN is likely powered by positron deposition ≳ 1yr after explosion. We also examine the geometry of the ejecta and show that the nebular line profiles of SN 2011dh indicate a roughly spherical explosion with aspherical components or clumps.