THE UNUSUAL TEMPORAL AND SPECTRAL EVOLUTION OF THE TYPE IIn SUPERNOVA 2011ht

We present very early UV to optical photometric and spectroscopic observations of the peculiar Type IIn supernova (SN) 2011ht in UGC 5460. The UV observations of the rise to peak are only the second ever recorded for a Type IIn SN and are by far the most complete. The SN, first classified as an SN impostor, slowly rose to a peak of M-V similar to -17 in similar to 55 days. In contrast to the similar to 2 mag increase in the upsilon-band light curve from the first observation until peak, the UV flux increased by >7 mag. The optical spectra are dominated by strong, Balmer emission with narrow peaks (FWHM similar to 600 km s(-1)), very broad asymmetric wings (FWHM similar to 4200 km s(-1)), and blueshifted absorption (similar to 300 km s(-1)) superposed on a strong blue continuum. The UV spectra are dominated by Fe II, Mg II, Si II, and Si III absorption lines broadened by similar to 1500 km s(-1). Merged X-ray observations reveal a L0.2-10 = (1.0 +/- 0.2) x 10(39) erg s(-1). Some properties of SN 2011ht are similar to SN impostors, while others are comparable to Type IIn SNe. Early spectra showed features typical of luminous blue variables at maximum and during giant eruptions. However, the broad emission profiles coupled with the strong UV flux have not been observed in previous SN impostors. The absolute magnitude and energetics (similar to 2.5 x 10(49) erg in the first 112 days) are reminiscent of normal Type IIn SN, but the spectra are of a dense wind. We suggest that the mechanism for creating this unusual profile could be a shock interacting with a shell of material that was ejected a year before the discovery of the SN.