Stream-Stream Collisions of Tidal Disruption Events
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 is a significant fraction of the Eddington accretion rate, 2% 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 mass flow rate drops significantly below the Eddington limit, with no unbound gas being produced when the mass flow rate drops to 1% of Eddington; we find however that the radiative efficiency increases slightly to 8% in these low mass flow rate cases. The effective radiation temperature and size of the photosphere is determined by the stream velocity and mass flow rate, 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 the mass flow rate , which can explain the rapidly changing photosphere sizes seen in TDE candidates such as PS1-10jh.
Size of the photosphere for the downstream gas becomes larger than the distance bewteen the collision point and the black hole when the mass flow rate of the stream is larger than ~10% of Eddington accretion rate. This is an effective way to produce the reprocessing layer in TDEs. Effective temperature of the gas from stream-stream collisions has strong angular dependences. It always decreases with increasing mass flow rate and photosphere size. These properties are quite different from emission from accretion disks. The downstream gas provides the source for the formation of accretion disks, which may be completely covered by the reprocessing layer. One interesting followup project is to calculate the global structures of the stream-stream collision and the formation of accretion disks after the collision.
