Nebular spectroscopy of the nearby type IIb supernova 2011dh

TL;DR

This study presents nebular spectra of supernova 2011dh over an extended period, revealing details about its progenitor, explosion mechanics, and late-time interactions, with new models confirming a low-mass giant origin and evidence of ejecta asymmetry.

Contribution

The paper provides detailed nebular spectral analysis of SN 2011dh, confirming the progenitor's low-mass giant nature and revealing late-time ejecta interaction and geometry.

Findings

Progenitor was a low-mass giant star (~13-15 M_sun).

SN ejecta interacted with circumstellar medium at late times.

SN powered by positron deposition after ~1 year.

Abstract

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.