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EVLA Observations of the Radio Evolution of SN 2011dh

We report on Expanded Very Large Array (EVLA) 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 \approx 0.1c, supporting the classification of the progenitor as a compact star (R_* \approx 10^11 cm). We find that the circumstellar density is consistent with a ρ \propto r^-2 profile. We determine that the progenitor shed mass at a constant rate of \approx 3 \times 10^-5 M_\odot / yr, assuming a wind velocity of 1000 km / s (values appropriate for a Wolf-Rayet star), or \approx 7 \times 10^-7 M_\odot / 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.