On the formation of a quasi-stationary twisted disc after a tidal disruption event

Abstract

We investigate misaligned accretion discs formed after tidal disruption events that occur when a star encounters a supermassive black hole. We employ the linear theory of warped accretion discs to find the shape of a disc for which the stream arising from the disrupted star provides a source of angular momentum that is misaligned with that of the black hole. For quasi-steady configurations we find that when the warp diffusion or propagation time is large compared to the local mass accretion time and/or the natural disc alignment radius is small, misalignment is favoured. These results have been verified using SPH simulations. We also simulated 1D model discs including gas and radiation pressure. As accretion rates initially exceed the Eddington limit the disc is initially advection dominated. Assuming the $α$ model for the disc, where it can be thermally unstable it subsequently undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from $\sim 1$ to $\sim 10^{-3}$ which is reflected in changes in the degree of disc misalignment at the stream impact location. For maximal black hole rotation and sufficiently large values of viscosity parameter $α> \sim 0.01-0.1$ the ratio of the disc inclination to that of the initial stellar orbit is estimated to be $0.1-0.2$ in the advection dominated state, while reaching of order unity in the low state. Misalignment descreases with decrease of $α$, but increases as the black hole rotation parameter decreases. Thus, it is always significant when the latter is small.