2013MNRAS.431.2599M

We present photometry and spectroscopy of the Type IIn supernova (SN) 2011ht, identified previously as a possible SN impostor. The light curve exhibits an abrupt transition from a well-defined ∼ 120d plateau to a steep bolometric decline, plummeting 4-5 mag in the optical and 2-3 mag in the infrared in only ∼ 10d. Leading up to peak brightness (MV = -17.4 mag), a hot emission-line spectrum exhibits strong signs of interaction with circumstellar material (CSM), in the form of relatively narrow P-Cygni features of Hi and Hei superimposed on broad Lorentzian wings. For the latter half of the plateau phase, the spectrum exhibits strengthening P-Cygni profiles of Feii, Caii and Hα. By day 147, after the plateau has ended, the SN entered the nebular phase, heralded by the appearance of forbidden transitions of [Oi], [Oii] and [Caii] over a weak continuum. At this stage, the light curve exhibits a low optical luminosity that is comparable to that of the most subluminous Type II-P supernovae, and a relatively fast visual wavelength decline that appeared to be significantly steeper than the 56Co decay rate. However, the total pseudo-bolometric decline, including the infrared luminosity, is consistent with 56Co decay, and implies a low 56Ni mass in the range 0.006-0.01M☉, near the lower end of the range exhibited by SNe II-P. We therefore characterize SN 2011ht as a core-collapse SN very similar to the peculiar SNe IIn 1994W and 2009kn. These three SNe appear to define a subclass, which are Type IIn based on their spectrum, but that also exhibit well-defined plateaus and produce low 56Ni yields. We therefore suggest Type IIn-P as a name for this subclass. The absence of observational signatures of high-velocity material from SNe IIn-P could be the result of an opaque shell at the shocked SN-CSM interface, which remains optically thick longer than the time-scale for the inner ejecta to cool and become transparent. Possible progenitors of SNe IIn-P, consistent with the available data, include 8-10M☉ stars, which undergo core collapse as a result of electron capture after a brief phase of enhanced mass loss, or more massive (M ≳ 25M☉) progenitors, which experience substantial fallback of the metal-rich radioactive ejecta. In either case, the energy radiated by these three SNe during their plateau (2-3x1049 erg for SN 2011ht) must be dominated by CSM interaction, and the subluminous tail is the result of low 56Ni yield.