Nebular spectroscopy of the nearby type IIb supernova 2011dh
  We present nebular spectra of the nearby Type IIb supernova (SN) 2011dh taken
between 201 and 678 days after core collapse. At these late times, SN 2011dh
exhibits strong emission lines including a very broad and persistent H{\alpha}
feature. New models of the nebular spectra confirm that the progenitor of SN
2011dh was a low-mass giant (M ~ 13 - 15 M_sun) that ejected ~0.07 M_sun of
56Ni and ~0.27 M_sun 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 (~2
yr) and not, for example, by a binary companion or a background source. We
present evidence for interaction between the expanding SN blastwave and a
circumstellar medium at late times and show that the SN is likely powered by
positron deposition by ~1 yr after explosion. We also examine the geometry of
the ejecta and show that SN 2011dh's nebular line profiles indicate a globally
spherical explosion with aspherical components or clumps.
