  In the past decade, several rapidly-evolving transients have been discovered
whose timescales and luminosities are not easily explained by traditional
supernovae (SN) models. The sample size of these objects has remained small
due, at least in part, to the challenge of detecting short timescale transients
with traditional survey cadences. Here we present the results from a search
within the Pan-STARRS1 Medium Deep Survey (PS1-MDS) for rapidly-evolving and
luminous transients. We identify 10 new transients with a time above
half-maximum of less than 12 days and -16.5 > M > -20 mag. This increases the
number of known events in this region of SN phase space by roughly a factor of
three. The median redshift of the PS1-MDS sample is z=0.275 and they all
exploded in star forming galaxies. In general, the transients possess faster
rise than decline timescale and blue colors at maximum light (g - r < -0.2).
Best fit blackbodies reveal photospheric temperatures/radii that expand/cool
with time and explosion spectra taken near maximum light are dominated by a
blue continuum, consistent with a hot, optically thick, ejecta. We find it
difficult to reconcile the short timescale, high peak luminosity (L > 10^43
erg/s), and lack of UV line blanketing observed in many of these transients
with an explosion powered mainly by the radioactive decay of Ni-56. Rather, we
find that many are consistent with either (1) cooling envelope emission from
the explosion of a star with a low-mass extended envelope which ejected very
little (<0.03 M_sun) radioactive material, or (2) a shock breakout within a
dense, optically thick, wind surrounding the progenitor star. After calculating
the detection efficiency for objects with rapid timescales in the PS1-MDS we
find a volumetric rate of 4800 - 8000 events/yr/Gpc^3 (4-7% of the
core-collapse SN rate at z=0.2).
