WHAT WAS SUPERNOVA 1988Z

We present spectroscopy and photometry of the type Il supernova SN 1988Z. This supernova was most unusual. Its characteristics include a high luminosity at maximum light, unusually slow fading at late times (0.005 mag d-1), and strong narrow emission lines arising from dense ( > 2 x 10(6) electrons CM-3) circumstellar material. Broad emission features with V approximately 10 000 km s-1 persisted for over a year and no absorption lines or P-Cygni profiles were observed, which is also exceptional for a type 11 supernova. We argue that SN 1988Z does not belong to an exotic new class of supernovae, but was a normal type II supernova which had a progenitor at the upper end of the mass range for red supergiants (up to 40 M.) with a dense, slow stellar wind which continued right up to core collapse. Most of the observed peculiarities can be understood in terms of an unusually massive stellar envelope. A pulsar formed during core collapse may be contributing to the slow decay in luminosity, but it is also possible that a substantial fraction of the excess energy comes from shocks.