  When a star is tidally disrupted by a supermassive black hole (BH), roughly
half of its mass falls back to the BH at super-Eddington rates. Being tenuously
gravitationally bound and unable to cool radiatively, only a small fraction
f_in << 1 of the returning debris will likely be incorporated into the disk and
accrete, with the vast majority instead becoming unbound in an outflow of
velocity ~1e4 km/s. This slow outflow spreads laterally, encasing the BH. For
months or longer, the outflow remains sufficiently neutral to block hard EUV
and X-ray radiation from the hot inner disk, which instead becomes trapped in a
radiation-dominated nebula. Ionizing nebular radiation heats the inner edge of
the ejecta to temperatures of T > few 1e4 K, converting the emission to
optical/near-UV wavelengths where photons more readily escape due to the lower
opacity. This can explain the unexpectedly low and temporally constant
effective temperatures of optically-discovered TDE flares. For BHs with
relatively high masses M_BH > 1e7 M_sun the ejecta can become ionized at an
earlier stage, or for a wider range of viewing angles, producing a TDE flare
which is instead dominated by thermal X-ray emission. We predict total radiated
energies consistent with those of observed TDE flares, and ejecta velocities
that agree with the measured emission line widths. The peak optical luminosity
for M_BH < 1e6 M_sun is suppressed due to adiabatic losses in the inner disk
wind, possibly contributing to the unexpected dearth of optical TDEs in
galaxies with low mass BHs. In the classical picture, where f_in ~ 1, TDEs
de-spin supermassive BHs and cap their maximum spins well below theoretical
accretion physics limits. This cap is greatly relaxed in our model, and
existing Fe K-alpha spin measurements provide suggestive preliminary evidence
that f_in < 1.

High Energy Astrophysical Phenomena