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Photometric Monitoring of Supernova (1Mar94) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 37, 1 March 1994) At early times, supernovae (SNe) are exciting events, brightening by ten magnitudes or more. Recent observations at KPNO show that the late phases also hold some thrills. SN type II are core collapse events that show strong hydrogen lines in their spectra at early times. The SN light curves reflect the structure of the stellar progenitors and reveal the underlying energy sources created in the explosion that power the expanding debris. The exponential decline of some SN II is attributed to the half-life of 56Ni (77 days) and 56Co (111 days), but this is not the whole story. Years after a supernova's initial brightening, photometric observations show a slower decline rate than seen at earlier times. Indeed, SN 1957D, and SN 1980K, have been re-discovered years after explosion, and SN 1986J (shown in the picture) and SN 1988Z are still visible. All of these are strong radio sources, too. What is powering the optical light several years after the explosion? [Figure not included] The power sources for the sustained optical emission have not been securely identified, though many theories exist. One option is the interaction of the remnant with the stellar wind of the progenitor star, which is also the favored interpretation of the radio emission. Other alternatives include long-lived radioactive isotopes such as 57Co or 44Ti, which could power the decline at very late times. A powerful pulsar, which may not be detected directly at large distances, is likely in core collapse events and could excite the debris. Optical light reflected off circumstellar dust at shock breakout (light echoes) is another possibility. Measuring the energy release in optical passbands over several years provides important information to distinguish among these mechanisms. R. Kirshner (CfA), L. Wells (NOAO), B. Schmidt (CfA), B. Leibundgut (ESO), P. Ruiz-Lapuente (CfA), and the late A. Porter (NOAO), have been monitoring SNe at very late times using the KPNO 2.1-m. The supernovae are generally declining very slowly, so monthly observations are adequate to monitor these objects. Observations under good seeing conditions for SN 1980K have reached down to 23.4 magnitudes in the B filter over 10 years past maximum light. The total flux emitted in the BVRI bands can be compared with estimates for the energy sources listed above. For example, SN1980K was observed to have a luminosity at the distance of NGC 6946 (the host galaxy) of 8 x 10**37 (D/7.5Mpc)**2 ergs s**-1. This probably rules out radioactive decay or light echoes as the energy source. Excitation by a pulsar would most likely result in narrow line emission, since only the inner material of the ejecta would be excited. Followup spectral observations show broad lines that may exclude this as the energy source. This leaves the interaction of the supernova ejecta with the remnant of the stellar wind of the progenitor star as the likely source of emission. Observations of old supernovae to faint limits with the 2.1-m thus are providing important clues to the last desperate stages of stellar evolution that precede stellar destruction.
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