Radio Observations of SN 1980K: Evidence for Rapid Presupernova Evolution

New observations of the supernova SN 1980K made with the VLA at 20 cm and 6 cm from 1994 April through 1996 October show that it has undergone a significant change in its radio emission evolution, dropping by a factor of ~2 below the flux density S ∝ t -0.73 power-law decline with time t observed earlier. However, although S at all observed frequencies has decreased significantly, its current spectral index of α = -0.42 ± 0.15 ( S ∝ ν +α ) is consistent with the previous spectral index of α = -0.60 −0.07 +0.04 . It is suggested that this decrease in emission may be due to the supernova (SN) shock entering a new region of the circumstellar material that has a lower density than that expected for a constant speed ( w ), constant mass-loss rate ( ) wind from the progenitor. If such an interpretation is correct, the difference in wind and shock speeds appears to indicate a significant evolution in the mass-loss history of the SN progenitor ~10 4 yr before explosion, with a change in circumstellar density (∝ / w ) occurring over a time span of ≲4 kyr. Such features could be explained in terms of a fast "blue-loop" evolutionary phase of a relatively massive pre-SN progenitor star. If so, we may, for the first time, provide a stringent constraint on the mass of the SN progenitor based solely on the SN's radio emission.