SN 2008ha: An Extremely Low Luminosity and Exceptionally Low Energy Supernova

We present ultraviolet, optical, and near-infrared photometry as well as optical spectra of the peculiar supernova (SN) 2008ha. SN 2008ha had a very low peak luminosity, reaching only M_V = -14.2 mag, and low line velocities of only ~2000 km s^-1 near maximum brightness, indicating a very small kinetic energy per unit mass of ejecta. Spectroscopically, SN 2008ha is a member of the SN 2002cx-like class of SNe, a peculiar subclass of SNe Ia however, SN 2008ha is the most extreme member, being significantly fainter and having lower line velocities than the typical member, which is already ~2 mag fainter and has line velocities ~5000 km s^-1 smaller (near maximum brightness) than a normal SN Ia. SN 2008ha had a remarkably short rise time of only ~10 days, significantly shorter than either SN 2002cx-like objects (~15 days) or normal SNe Ia (~19.5 days). The bolometric light curve of SN 2008ha indicates that SN 2008ha peaked at L _peak = (9.5 ± 1.4) × 10⁴⁰ erg s^-1, making SN 2008ha perhaps the least luminous SN ever observed. From its peak luminosity and rise time, we infer that SN 2008ha generated (3.0 ± 0.9) × 10^-3 M _sun of ⁵⁶Ni, had a kinetic energy of ~2 × 10⁴⁸ erg, and ejected 0.15 M _sun of material. The host galaxy of SN 2008ha has a luminosity, star formation rate, and metallicity similar to those of the Large magellanic Cloud. We classify three new (and one potential) members of the SN 2002cx-like class, expanding the sample to 14 (and one potential) members. The host-galaxy morphology distribution of the class is consistent with that of SNe Ia, Ib, Ic, and II. Several models for generating low-luminosity SNe can explain the observations of SN 2008ha however, if a single model is to describe all SN 2002cx-like objects, deflagration of carbon-oxygen white dwarfs, with SN 2008ha being a partial deflagration and not unbinding the progenitor star, is preferred. The rate of SN 2008ha-like events is ~10% of the SN Ia rate, and in the upcoming era of transient surveys, several thousand similar objects may be discovered, suggesting that SN 2008ha may be the tip of a low-luminosity transient iceberg.