Type IIn Supernovae

Abstract

In this chapter discusses supernovae of Type IIn (SN IIn), the distinguishing characteristic of which is the presence of narrow spectral lines of hydrogen, indicative of circumstellar interaction. The circumstellar medium (CSM) arises from winds of the progenitor star and impulsive presupernova mass ejections. SN IIn tend to be brighter than supernovae of Type IIP (Chapter “Type IIP Supernovae”) and they display a wide variety of light-curve shapes including very extended light curves in some cases. SN IIn can be strong radio and X-ray sources and often become strong infrared sources owing to emission by dust grains, some of which may have been present in the presupernova CSM and others may have formed in the ejecta. The extant spectropolarization data indicate that the CSM and perhaps the ejecta tend to be significantly asymmetric. This chapter also discusses supernova impostors, stars that eject massive shells, brighten dramatically, and display narrow spectral lines of hydrogen similar to those of SN IIn. These outbursts do not, however, fundamentally transform the nature of the underlying star. In some cases it is not clear whether a particular event is an impostor or a supernova. A subset of SN IIn, the SN 1994-likes, show symmetric scattering lines of Hα with a blue absorption at about 600 km s-1 that is nearly constant in time and among similar events.