Some radio supernovae have been monitored for several decades after the explosion. They become SNRs some 100-1000 years later. We do not know what happens during the transition from RSN to SNR, i.e., in the transition from interacting with the circumstellar material left over from the progenitor to interacting with the general interstellar medium.
The radio behavior during the SN phase diagnoses the history of mass loss from the progenitor, since the emission comes from the shock between the ejecta and the CSM. A sharp intensity decrease indicates the outer edge of the CSM, giving a timescale for the progenitor's slow wind, and yielding an estimate for the total mass loss for that star. This can be compared with stellar evolution and ISM enrichment theories, a comparison that is particularly interesting if the SN was observed optically. This has been done so far for only four SNe. The difficulty lies in tracking supernovae to very low flux levels; SN 1961V in NGC 1058 ( = 518 km/s) was recovered at about 0.2 mJy at 20cm To reach a similar level in the Virgo cluster (about a factor of two further) we need 10 microJy rms noise, achievable in about 1 hr with the enhanced sensitivity; even with several pointings, the dozens of optically-known SNe in that cluster could be detected easily.
SNRs indicate recent massive star formation (stars that blew up years ago) and are the main source of the high energy particles (cosmic rays) that form the diffuse synchrotron emission in galactic disks and halos. They are important both in terms of energy input, disturbing the ISM, creating H I ``bubbles'' and possibly triggering further star formation, and as sources of metals in galactic disks. The current state of SNR detection is illustrated by recent observations of M33, where some 50 SNR were detected at an rms sensitivity of 50 microJy at 1.4 GHz. The cumulative distribution of SNRs as a function of flux density suggests either that incompleteness sets in at 0.5 mJy, or that the source counts may turn over there. Observations with a factor 10 higher sensitivity could be achieved in less than 2 hrs with the proposed 2.4 GHz system, and would settle this question beyond doubt. If the steeper power law is correct, this would yield a factor 20 more sources, or some 1000 SNRs. Confusion will be significant but one should be able to determine the supernova rate statistically with such large numbers. As well as obtaining a crucial parameter for models of energy input into the ISM, galactic abundances, etc. much more accurately, this would also give the ``turnover time" for replenishing the cosmic rays in the galactic disk, which with more SNRs would be embarrassingly short. As well as such detailed studies, one could extend such observations to Mpc with 2-hr integrations, yielding more galaxies, and examining the dependence of SN rates and the available pool of particle accelerators on galaxy type and activity.
Resolving SNRs yields much more information than simple detections-it lets us distinguish SNRs from background sources by their morphology and also adds important physical information about the SNRs. For instance, the size gives an indication of the age of a remnant, and the relationship between size and surface brightness (the - relation) should also reflect the evolution of the source. The results of such studies in the Milky Way are muddled, at least in part, because of inhomogeneous samples and uncertainties in SNR distances. Studying SNRs in another galaxy provides better statistics (more SNRs, some indication of dependence on galactic environment, much better understanding of selection effects) and a common distance for all SNRs seen. Resolved images also give SNR types (filled, shell, or mixed), giving some indication of SNR histories and current power sources (pulsar vs. shock). Ideally one would like sub-parsec resolution at 0.6-2.4 GHz. The current A configuration would be useful for the larger sources, but the A+ configuration would be much better, giving a 11cm resolution of pc at the distance of M33 (800 kpc).
These observations require high sensitivity over the entire primary beam at a low frequency. We need a high-sensitivity system at some frequency between 0.6 and 2.4 GHz; which frequency is not so important, nor does one need continuous coverage. Low frequencies are best because these sources have fairly steep spectra ( to ). One should also be able to image the entire primary beam at the highest available resolutions, to cover an entire galaxy or more with one pointing.