  Transient surveys have recently discovered a class of supernovae (SNe) with
extremely rapidly declining light curves. These events are also often
relatively faint, especially compared to Type Ia SNe. The common explanation
for these events involves a weak explosion, producing a radioactive outflow
with small ejected mass and kinetic energy (M ~ 0.1 Msun and E ~ 0.1 B,
respectively), perhaps from the detonation of a helium shell on a white dwarf.
We argue, in contrast, that these events may be Type Ib/c SNe with typical
masses and energies (M ~ 3 Msun, E ~ 1 B), but which ejected very little
radioactive material. In our picture, the light curve is powered by the
diffusion of thermal energy deposited by the explosion shock wave, and the
rapid evolution is due to recombination, which reduces the opacity and results
in an "oxygen-plateau" light curve. Using a radiative transfer code, we
generate synthetic spectra and light curves and demonstrate that this model can
reasonably fit the observations of one event, SN 2010X. Similar models may
explain the features of other rapidly evolving SNe such as SN 2002bj and SN
2005ek. SNe such as these require stripped-envelope progenitors with rather
large radii (R ~ 20 Rsun), which may originate from a mass loss episode
occurring just prior to explosion.
