  Stream-stream collisions play an important role for the circularization of
highly eccentric streams resulting from tidal disruption events (TDEs). We
perform three dimensional radiation hydrodynamic simulations to show that
stream collisions can contribute significant optical and ultraviolet light to
the flares produced by TDEs, and can sometimes explain the majority of the
observed emission. Our simulations focus on the region near the radiation
pressure dominated shock produced by a collision and track how the kinetic
energy of the stream is dissipated by the associated shock. When the mass flow
rate of the stream $\dot{M}$ is a significant fraction of the Eddington
accretion rate, $\gtrsim2\%$ of the initial kinetic energy is converted to
radiation directly as a result of the collision. In this regime, the collision
redistributes the specific kinetic energy into the downstream gas and more than
$16\%$ of the mass can become unbound. The fraction of unbound gas decreases
rapidly as $\dot{M}$ drops significantly below the Eddington limit, with no
unbound gas being produced when $\dot{M}$ drops to $1\%$ of Eddington; we find
however that the radiative efficiency increases slightly to $\lesssim 8\%$ in
these low $\dot{M}$ cases. The effective radiation temperature and size of the
photosphere is determined by the stream velocity and $\dot{M}$, which we find
to be a few times $10^4$~K and $10^{14}$~cm in our calculations, comparable to
the inferred values of some TDE candidates. The photosphere size is directly
proportional to $\dot{M}$, which can explain the rapidly changing photosphere
sizes seen in TDE candidates such as PS1-10jh.

High Energy Astrophysical Phenomena