SN 2011ht: confirming a class of interacting supernovae with plateau light curves (Type IIn-P)

We present photometry and spectroscopy of the Type IIn supernova (SN) 2011ht,\nidentified previously as a SN impostor. The light curve exhibits an abrupt\ntransition from a well-defined ~120 day plateau to a steep bolometric decline.\nLeading up to peak brightness, a hot emission-line spectrum exhibits signs of\ninteraction with circumstellar material (CSM), in the form of relatively narrow\nP-Cygni features of H I and He I superimposed on broad Lorentzian wings. For\nthe remainder of the plateau phase the spectrum exhibits strengthening P-Cygni\nprofiles of Fe II, Ca II, and H-alpha. By day 147, after the plateau has ended,\nthe SN entered the nebular phase, heralded by the appearance of forbidden\ntransitions of [O I], [O II], and [Ca II] over a weak continuum. At this stage,\nthe light curve exhibits a low luminosity that is comparable to that\nsub-luminous Type II-P supernovae, and a relatively fast visual-wavelength\ndecline that is significantly steeper than the Co-56 decay rate. However, the\ntotal bolometric decline, including the IR luminosity, is consistent with Co-56\ndecay, and implies a low Ni-56 mass of ~0.01 M(Sun). We therefore characterize\nSN 2011ht as a bona-fide core-collapse SN very similar to the peculiar SNe IIn\n1994W and 2009kn. These three SNe define a subclass, which are Type IIn based\non their spectrum, but that also exhibit well-defined plateaus and produce low\nNi-56 yields. We therefore suggest Type IIn-P as a name for this subclass.\nPossible progenitors of SNe IIn-P, consistent with the available data, include\n8-10 M(Sun) stars, which undergo core collapse as a result of electron capture\nafter a brief phase of enhanced mass loss, or more massive M>25 M(Sun)\nprogenitors, which experience substantial fallback of the metal-rich\nradioactive ejecta. In either case, the energy radiated by these three SNe\nduring their plateau must be dominated by CSM interaction (abridged).\n