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We report on Expanded Very Large Array (EVLA) observations of the Type IIbsupernova 2011dh, performed over the first 100 days of its evolution andspanning 1-40 GHz in frequency. The radio emission is well-described by theself-similar propagation of a spherical shockwave, generated as the supernovaejecta interact with the local circumstellar environment. Modeling thisemission with a standard synchrotron self-absorption (SSA) model gives anaverage expansion velocity of v \approx 0.1c, supporting the classification ofthe progenitor as a compact star (R_* \approx 10^11 cm). We find that thecircumstellar density is consistent with a {\rho} \propto r^-2 profile. Wedetermine that the progenitor shed mass at a constant rate of \approx 3 \times10^-5 M_\odot / yr, assuming a wind velocity of 1000 km / s (values appropriatefor 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 themass-loss rate assume a converted fraction of kinetic to magnetic energydensity of {\epsilon}_B = 0.1. Although optical imaging shows the presence of aYSG, the rapid optical evolution and fast expansion argue that the progenitoris a more compact star - perhaps a companion to the YSG. Furthermore, theexcellent agreement of the radio properties of SN 2011dh with the SSA modelimplies that any YSG companion is likely in a wide, non-interacting orbit.

EVLA Observations of the Radio Evolution of SN 2011dh