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 has beenknown that the debris forms a precessing stream, which may collide with itself.The stream collision is a key process determining the subsequent evolution ofthe stellar debris: if the orbital energy is efficiently dissipated, the debriswill eventually form a circular disk (or torus). In this paper, we havenumerically studied such stream collision resulting from the encounter betweena 10^6 Msun black hole and a 1 Msun normal star with a pericenter radius of 100Rsun. A simple treatment for radiative cooling has been adopted for bothoptically thick and thin regions. We have found that approximately 10 to 15% ofthe initial kinetic energy of the streams is converted into thermal energyduring the collision. The angular momentum of the incoming stream is increasedby a factor of 2 to 3, and such increase, together with the decrease in kineticenergy, significantly helps the circularization process. Initial luminosityburst due to the collision may reach as high as 10^41 erg/sec in 10^4 sec,after which the luminosity increases again (but slowly this time) to a steadyvalue of a few 10^40 erg/sec in a few times of 10^5 sec. The radiation from thesystem is expected to be close to Planckian with effective temperature of\~10^5K.Comment: 19 pages including 12 figures; Accepted for publication in Ap