What are radio-loud supernovae?

Several supernovae have now been discovered serendipitously as strong radio sources even though they were not seen optically at the time of explosion. SN1986J in NGC891 (Rupen et al. 1987) and SN1978K in NGC1313 (Ryder et al. 1993) are two examples. The progenitors of these radio-loud supernovae appear to be massive (>30 M) red giants in which a high rate of mass loss continued up to the moment of explosion. The radio emission arises from interaction of the supernova shock wave with this dense stellar wind.

It is important to distinguish between radio supernovae and classical supernova remnants. The emission from radio supernovae arises in material directly surrounding the star, and peaks between a few months and a few years after explosion (after which it declines slowly for several decades). Radio supernovae can be far more powerful radio sources than Galactic supernova remnants (SN1986J at its peak was over a thousand times more powerful than Cas A).

A more systematic investigation of radio-loud supernovae (RSNe) is likely to be important for several reasons:

• If some radio-loud supernovae are optically dim or invisible (as SN1986J appears to have been), and if these are also the most massive stars (with a corresponding large yield of processed stellar material), then there are important consequences for our understanding of the chemical enrichment history of galaxies.
• Because of their high radio luminosity and slow decay time, RSNe may be significant contributors to the total radio flux of starburst galaxies, and hence of interest in studying the FIR-radio correlation.
• Long-term monitoring of RSNe at several frequencies may yield valuable clues to the circumstellar environment of massive stars.
• If RSNe are detected sufficiently early, VLBI measurements of the expanding shell offer the possibility of a direct distance determination, and hence a measurement of the Hubble constant (e.g. Bartel 1988).

© Copyright Astronomical Society of Australia 1997