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Stream-stream collisions play an important role for the circularization ofhighly eccentric streams resulting from tidal disruption events (TDEs). Weperform three dimensional radiation hydrodynamic simulations to show thatstream collisions can contribute significant optical and ultraviolet light tothe flares produced by TDEs, and can sometimes explain the majority of theobserved emission. Our simulations focus on the region near the radiationpressure dominated shock produced by a collision and track how the kineticenergy of the stream is dissipated by the associated shock. When the mass flowrate of the stream $\dot{M}$ is a significant fraction of the Eddingtonaccretion rate, $\gtrsim2\%$ of the initial kinetic energy is converted toradiation directly as a result of the collision. In this regime, the collisionredistributes the specific kinetic energy into the downstream gas and more than$16\%$ of the mass can become unbound. The fraction of unbound gas decreasesrapidly as $\dot{M}$ drops significantly below the Eddington limit, with nounbound gas being produced when $\dot{M}$ drops to $1\%$ of Eddington; we findhowever that the radiative efficiency increases slightly to $\lesssim 8\%$ inthese low $\dot{M}$ cases. The effective radiation temperature and size of thephotosphere is determined by the stream velocity and $\dot{M}$, which we findto be a few times $10^4$~K and $10^{14}$~cm in our calculations, comparable tothe inferred values of some TDE candidates. The photosphere size is directlyproportional to $\dot{M}$, which can explain the rapidly changing photospheresizes seen in TDE candidates such as PS1-10jh.

Prompt Radiation and Mass Outflows from the Stream-Stream Collisions of Tidal Disruption Events