What was supernova 1988Z?

Abstract

We present spectroscopy and photometry of the type II 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 (>2xl06 electrons cm-3) circumstellar material. Broad emission features with V∼ 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 II 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 Mo) 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.