The Radio Recovery of SN 1970G: The Continuing Radio Evolution of SN 1970G

Using the Very Large Array, we have detected radio emission from the site of SN 1970G in the Sc galaxy M101. These observations are 31 years after the supernova event, making SN 1970G the longest monitored radio supernova. With flux densities of 0.12 +/- 0.020 mJy at 6 cm and 0.16 +/- 0.015 mJy at 20 cm, the spectral index of -0.24 +/- 0.20 appears to have flattened somewhat when compared with the previously reported value of -0.56 +/- 0.11, taken in 1990. The radio emission at 20 cm has decayed since the 1990 observations with a power-law index of beta_20cm = -0.28 +/- 0.13. We discuss the radio properties of this source and compare them to those of other Type II radio supernovae.Comment: 11 pages, 1 table and 2 figures; To appear in Astrophysical Journal Letter

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

New observations of SN 1980K made with the VLA at 20 and 6 cm from 1994 April through 1996 October show that the supernova (SN) has undergone a significant change in its radio emission evolution, dropping by a factor of ~2 below the flux density S \propto 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 \alpha= -0.42\pm0.15 (S \propto \nu^{+\alpha}) is consistent with the previous spectral index of \alpha=-0.60_{-0.07}^{+0.04}. It is suggested that this decrease in emission may be due to the SN shock entering a new region of the circumstellar material which has a lower density than that expected for a constant speed (w), constant mass-loss rate (Mdot) 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 years before explosion, with a change in circumstellar density (\propto Mdot/w) occurring over a time span of \lesssim 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.Comment: 22 pages, 3 figures, to appear in Ap