ALBERT

Description: 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 M BH 2 x 10 6 M , the spin-sensitive behaviour associated with efficient cooling may be realized.

Description: We study accretion processes for tidally disrupted stars approaching supermassive black holes on bound orbits, by performing three-dimensional smoothed particle hydrodynamics simulations with a pseudo-Newtonian potential. We find that there is a critical value of the orbital eccentricity below which all the stellar debris remains bound to the black hole. For high but subcritical eccentricities, all the stellar mass is accreted on to the black hole in a finite time, causing a significant deviation from the canonical t –5/3 mass fallback rate. When a star is on a moderately eccentric orbit and its pericentre distance is deeply inside the tidal disruption radius, there can be several orbit crossings of the debris streams due to relativistic precession. This dissipates orbital energy in shocks, allowing for rapid circularization of the debris streams and formation of an accretion disc. The resultant accretion rate greatly exceeds the Eddington rate and differs strongly from the canonical rate of t –5/3 . By contrast, there is little dissipation due to orbital crossings for the equivalent simulation with a purely Newtonian potential. This shows that general relativistic precession is crucial for accretion disc formation via circularization of stellar debris from stars on moderately eccentric orbits.