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 th at 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 encount er between a 10(6) M. black hole and a 1 M. normal star with a pericenter r adius of 100 R.. A simple treatment for radiative cooling has been adopted for both optically thick and optically thin regions. We have found that app roximately 10% to 15% of the initial kinetic energy of the streams is conve rted into thermal energy during the collision. The spread in angular moment um of the incoming stream is increased by a factor of 2 to 3, and such an i ncrease, together with the decrease in kinetic energy, significantly helps the circularization process. The initial luminosity burst produced by the c ollision 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 f ew 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 simi lar to 10(5) K.