We study the circularization of tidally disrupted stars on bound orbits around spinning supermassive black holes by performing 3D smoothed particle hydrodynamic simulations with post-Newtonian corrections. Our simulations reveal that debris circularization depends sensitively on the efficiency of radiative cooling. There are two stages in debris circularization if radiative cooling is inefficient: first, the stellar debris streams self-intersect due to relativistic apsidal precession; shocks at the intersection points thermalize orbital energy and the debris forms a geometrically thick, ring-like structure around the black hole. The ring rapidly spreads via viscous diffusion, leading to the formation of a geometrically thick accretion disc. In contrast, if radiative cooling is efficient, the stellar debris circularizes due to self-intersection shocks and forms a geometrically thin ring-like structure. In this case, the dissipated energy can be emitted during debris circularization as a precursor to the subsequent tidal disruption flare. The circularization time-scale is remarkably long in the radiatively efficient cooling case, and is also sensitive to black hole spin. Specifically, Lense-Thirring torques cause dynamically important nodal precession, which significantly delays debris circularization. On the other hand, nodal precession is too slow to produce observable signatures in the radiatively inefficient case. Since the stellar debris is optically thick and its photon diffusion time is likely longer than the time-scale of shock heating, our inefficient cooling scenario is more generally applicable in eccentric tidal disruption events (TDEs). However, in parabolic TDEs for MBH ≳ 2 × 106M⊙, the spin-sensitive behaviour associated with efficient cooling may be realized.
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ACKNOWLEDGEMENTS The authors thank to the anonymous referee for fruitful comments and suggestions. KH is grateful to Atsuo. T Okazaki and Jongsoo Kim for their helpful discussions and continuous encouragement. KH would also like to thank the Kavli Institute for Theoretical Physics (KITP) for their hospitality and support during the program on A Universe of Black Holes. During the completion of this paper, we became aware of complementary numerical simulations by Amaro-Seoane et al. (in preparation), but both efforts have proceeded independently. Numerical simulations and data reductions reported here were performed by using a high-performance computing cluster (Polaris) at the Korea Astronomy and Space Science Institute and by using computer facilities at Department of Astronomy, Kyoto University, and Harvard Smithsonian Center for Astrophysics, Harvard University. This work was supported in part by the research grants of the Chungbuk National University in 2015 and Korea Astronomy and Space Science Institute in 2016 [KH], the Alfred P. Sloan Foundation through a grant to Brian Metzger [NS], and NSF grant AST-1312034 [AL].
© 2016 The Authors.
- Black hole physics
- Gravitational waves
- accretion discs