A Continuum of H- to He-Rich Tidal Disruption Candidates With a
  Preference for E+A Galaxies
  We present the results of a Palomar Transient Factory (PTF) archival search
for blue transients which lie in the magnitude range between "normal"
core-collapse and superluminous supernovae (i.e. with
$-21\,{\leq}M_{R\,(peak)}\,{\leq}-19$). Of the six events found after excluding
all interacting Type~IIn and Ia-CSM supernovae, three (PTF09ge, 09axc and
09djl) are coincident with the centers of their hosts, one (10iam) is offset
from the center, and for two (10nuj and 11glr) a precise offset can not be
determined. All the central events have similar rise times to the He-rich tidal
disruption candidate PS1-10jh, and the event with the best-sampled light curve
also has similar colors and power-law decay. Spectroscopically, PTF09ge is
He-rich, while PTF09axc and 09djl display broad hydrogen features around peak
magnitude. All three central events are in low star-formation hosts, two of
which are E+A galaxies. Our spectrum of the host of PS1-10jh displays similar
properties. PTF10iam, the one offset event, is different photometrically and
spectroscopically from the central events and its host displays a higher star
formation rate. Finding no obvious evidence for ongoing galactic nuclei
activity or recent star formation, we conclude that the three central
transients likely arise from the tidal disruption of a star by a super-massive
black hole. We compare the spectra of these events to tidal disruption
candidates from the literature and find that all of these objects can be
unified on a continuous scale of spectral properties. The accumulated evidence
of this expanded sample strongly supports a tidal disruption origin for this
class of nuclear transients.
