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 0.1 c , supporting the classification of the progenitor as a compact star ( R _ { \star } \approx 10 ^ { 11 } ~ { } cm ) .
We find that the circumstellar density is consistent with a \rho \propto r ^ { -2 } profile .
We determine that the progenitor shed mass at a constant rate of \approx 3 \times 10 ^ { -5 } ~ { } M _ { \sun } ~ { } yr ^ { -1 } , assuming a wind velocity of 1000 ~ { } km~ { } s ^ { -1 } ( values appropriate for a Wolf-Rayet star ) , or \approx 7 \times 10 ^ { -7 } ~ { } M _ { \sun } ~ { } yr ^ { -1 } assuming 20 ~ { } km~ { } s ^ { -1 } ( 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 \epsilon _ { 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 .