A Hubble Space Telescope snapshot survey of nearby supernovae

We present photometry of 13 recent supernovae (SNe) recovered in a Hubble Space Telescope Snapshot program and tie the measurements to earlier ground-based observations in order to study the late-time evolution of the SNe. Many of the ground-based measurements are previously unpublished and were made primarily with a robotic telescope, the Katzman Automatic Imaging Telescope. Evidence for circumstellar interaction is common among the core-collapse SNe. Late-time decline rates for Type IIn SNe are found to span a wide range, perhaps because of differences in circumstellar interaction. An extreme case, SN IIn 1995N, declined by only 1.2 mag in V over about 4 yr following discovery. Template images of some SNe must therefore be obtained many years after the explosion if contamination from the SN itself is to be minimized. Evidence is found against a previous hypothesis that the Type IIn SN 1997bs was actually a superoutburst of a luminous blue variable star. The peculiar SN Ic 1997ef, a "hypernova," declined very slowly at late times. The decline rate of the SN Ia 2000cx decreased at late times, but this is unlikely to have been caused by a light echo.