SN 2008ha: An Extremely Low Luminosity and Extremely 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 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 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) x 10^40 ergs/s, 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) x 10^-3 M_sun of 56Ni, had a kinetic
energy of ~2 x 10^48 ergs, and ejected 0.15 M_sun of material. 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, either electron capture in Ne-Mg white dwarfs causing a core collapse,
or deflagration of C-O white dwarfs with SN 2008ha being a partial deflagration
and not unbinding the progenitor star, are preferred. Abridged.
