The Stream‐Stream Collision after the Tidal Disruption of a Star around a Massive Black Hole

A star can be tidally disrupted around a massive black hole. It is known that the debris forms a precessing stream, which may collide with itself. The stream collision is a key process in determining the subsequent evolution of the stellar debris: if the orbital energy is efficiently dissipated, the debris will eventually form a circular disk (or torus). In this paper, we have numerically studied such a stream collision resulting from the encounter between a 10 6 M ☉ black hole and a 1 M ☉ normal star with a pericenter radius of 100 R ☉ . A simple treatment for radiative cooling has been adopted for both optically thick and optically thin regions. We have found that approximately 10% to 15% of the initial kinetic energy of the streams is converted into thermal energy during the collision. The spread in angular momentum of the incoming stream is increased by a factor of 2 to 3, and such an increase, together with the decrease in kinetic energy, significantly helps the circularization process. The initial luminosity burst produced by the collision may reach as high as 10 41 ergs s -1 in 10 4 s, after which the luminosity increases again (but slowly this time) to a steady value of a few 10 40 ergs s -1 in a few times 10 5 s. The radiation from the system is expected to be close to Planckian, with an effective temperature of ~10 5 K.