Spectral features of tidal-disruption candidates and alternative origins
  for such transient flares
  UV and optically selected candidates for stellar tidal disruption events
(TDE) often exhibit broad spectral features (HeII emission, H$\alpha$ emission,
or absorption lines) on a blackbody-like continuum (1e4K<T<1e5K). The lines
presumably emit from TDE debris or circumnuclear clouds photoionized by the
flare. Line velocities however are much lower than expected from a stellar
disruption by supermassive black hole (SMBH), and are somewhat faster than
expected for the broad line region (BLR) clouds of a persistently active
galactic nucleus (AGN). The distinctive spectral states are not strongly
related to observed luminosity and velocity, nor to SMBH mass estimates. We use
exhaustive photoionization modelling to map the domain of fluxes and cloud
properties that yield (e.g.) a He-overbright state where a large
HeII(4686A)/H$\alpha$ line-ratio creates an illusion of helium enrichment.
Although observed line ratios occur in a plausible minority of cases, AGN-like
illumination can not reproduce the observed equivalent widths. We therefore
propose to explain these properties by a light-echo photoionization model: the
initial flash of a hot blackbody (detonation) excites BLR clouds, which are
then seen superimposed on continuum from a later, expanded, cooled stage of the
central luminous source. The implied cloud mass is substellar, which may be
inconsistent with a TDE. Given these and other inconsistencies with TDE models
(e.g. host-galaxies distribution) we suggest to also consider alternative
origins for these nuclear flares, which we briefly discuss (e.g. nuclear
supernovae and starved/subluminous AGNs).
