SN 2008ha: AN EXTREMELY LOW LUMINOSITY AND EXCEPTIONALLY LOW ENERGY SUPERNOVA

We present ultraviolet, optical, and near-infrared photometry as well as\noptical spectra of the peculiar supernova (SN) 2008ha. SN 2008ha had a very low\npeak luminosity, reaching only M_V = -14.2 mag, and low line velocities of only\n~2000 km/s near maximum brightness, indicating a very small kinetic energy per\nunit mass of ejecta. Spectroscopically, SN 2008ha is a member of the SN\n2002cx-like class of SNe, a peculiar subclass of SNe Ia; however, SN 2008ha is\nthe most extreme member, being significantly fainter and having lower line\nvelocities than the typical member, which is already ~2 mag fainter and has\nline velocities ~5000 km/s smaller (near maximum brightness) than a normal SN\nIa. SN 2008ha had a remarkably short rise time of only ~10 days, significantly\nshorter than either SN 2002cx-like objects (~15 days) or normal SNe Ia (~19.5\ndays). The bolometric light curve of SN 2008ha indicates that SN 2008ha peaked\nat L_peak = (9.5 +/- 1.4) x 10^40 ergs/s, making SN 2008ha perhaps the least\nluminous SN ever observed. From its peak luminosity and rise time, we infer\nthat SN 2008ha generated (3.0 +/- 0.9) x 10^-3 M_sun of 56Ni, had a kinetic\nenergy of ~2 x 10^48 ergs, and ejected 0.15 M_sun of material. We classify\nthree new (and one potential) members of the SN 2002cx-like class, expanding\nthe sample to 14 (and one potential) members. The host-galaxy morphology\ndistribution of the class is consistent with that of SNe Ia, Ib, Ic, and II.\nSeveral models for generating low-luminosity SNe can explain the observations\nof SN 2008ha; however, if a single model is to describe all SN 2002cx-like\nobjects, either electron capture in Ne-Mg white dwarfs causing a core collapse,\nor deflagration of C-O white dwarfs with SN 2008ha being a partial deflagration\nand not unbinding the progenitor star, are preferred. Abridged.\n