2012ApJ...750L..40K

We report on Expanded Very Large Array observations of the Type IIb supernova 2011dh, performed over the first 100 days of its evolution and spanning 1-40 GHz in frequency. The radio emission is well described by the self-similar propagation of a spherical shockwave, generated as the supernova ejecta interact with the local circumstellar environment. Modeling this emission with a standard synchrotron self-absorption (SSA) model gives an average expansion velocity of v ~ 0.1c, supporting the classification of the progenitor as a compact star (R_{sstarf}~ 10¹¹ cm). We find that the circumstellar density is consistent with a ρ∝r ^–2 profile. We determine that the progenitor shed mass at a constant rate of ~3 x10^–5 M_☉/yr, assuming a wind velocity of 1000 km/s (values appropriate for a Wolf-Rayet star), or ~7x10^–7 M_☉/yr assuming 20 km/s (appropriate for a yellow supergiant [YSG] star). Both values of the mass-loss rate assume a converted fraction of kinetic to magnetic energy density of ε_B= 0.1. Although optical imaging shows the presence of a YSG, the rapid optical evolution and fast expansion argue that the progenitor is a more compact star–perhaps a companion to the YSG. Furthermore, the excellent agreement of the radio properties of SN 2011dh with the SSA model implies that any YSG companion is likely in a wide, non-interacting orbit.