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

Abstract

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.