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,
identified previously as a SN impostor. The light curve exhibits an abrupt
transition from a well-defined ~120 day plateau to a steep bolometric decline.
Leading up to peak brightness, a hot emission-line spectrum exhibits signs of
interaction with circumstellar material (CSM), in the form of relatively narrow
P-Cygni features of H I and He I superimposed on broad Lorentzian wings. For
the remainder of the plateau phase the spectrum exhibits strengthening P-Cygni
profiles of Fe II, Ca II, and H-alpha. By day 147, after the plateau has ended,
the SN entered the nebular phase, heralded by the appearance of forbidden
transitions of [O I], [O II], and [Ca II] over a weak continuum. At this stage,
the light curve exhibits a low luminosity that is comparable to that
sub-luminous Type II-P supernovae, and a relatively fast visual-wavelength
decline that is significantly steeper than the Co-56 decay rate. However, the
total bolometric decline, including the IR luminosity, is consistent with Co-56
decay, and implies a low Ni-56 mass of ~0.01 M(Sun). We therefore characterize
SN 2011ht as a bona-fide core-collapse SN very similar to the peculiar SNe IIn
1994W and 2009kn. These three SNe define a subclass, which are Type IIn based
on their spectrum, but that also exhibit well-defined plateaus and produce low
Ni-56 yields. We therefore suggest Type IIn-P as a name for this subclass.
Possible progenitors of SNe IIn-P, consistent with the available data, include
8-10 M(Sun) stars, which undergo core collapse as a result of electron capture
after a brief phase of enhanced mass loss, or more massive M>25 M(Sun)
progenitors, which experience substantial fallback of the metal-rich
radioactive ejecta. In either case, the energy radiated by these three SNe
during their plateau must be dominated by CSM interaction (abridged).
