[Paper Review] Disc formation from tidal disruption of stars on eccentric orbits by Kerr black holes using GRSPH

David Liptai, Daniel J. Price+2 more|arXiv (Cornell University)|Oct 22, 2019

Astrophysical Phenomena and ObservationsPhysics and Astronomy2 references19 citations

TL;DR

This study uses 3D general relativistic smoothed particle hydrodynamics (GRSPH) in a Kerr black hole spacetime to simulate tidal disruption of stars on eccentric orbits, showing that black hole spin-induced nodal precession delays disc formation by only a short time, while radiative cooling shapes the remnant disc into a narrow, rapidly precessing ring. The simulations reveal energy dissipation rates up to ~10^47 erg s⁻¹ for deep encounters, with disc structure highly sensitive to cooling efficiency.

ABSTRACT

We perform 3D general relativistic smoothed particle hydrodynamics (GRSPH) simulations of tidal disruption events involving 1 $M_\odot$ stars and $10^6 M_\odot$ rotating supermassive black holes. We consider stars on initially elliptical orbits both in, and inclined to, the black hole equatorial plane. We confirm that stream-stream collisions caused by relativistic apsidal precession rapidly circularise the disrupted material into a disc. For inclined trajectories we find that nodal precession induced by the black hole spin (i.e. Lense-Thirring precession) inhibits stream-stream collisions only in the first orbit, merely causing a short delay in forming a disc, which is inclined to the black hole equatorial plane. We also investigate the effect of radiative cooling on the remnant disc structure. We find that with no cooling a thick, extended, slowly precessing torus is formed, with a radial extent of 5 au (for orbits with a high penetration factor). Radiatively efficient cooling produces a narrow, rapidly precessing ring close to pericentre. We plot the energy dissipation rate, which tracks the pancake shock, stream-stream collisions and viscosity. We compare this to the effective luminosity due to accretion onto the black hole. We find energy dissipation rates of $\sim10^{45}$ erg s$^{-1}$ for stars disrupted at the tidal radius, and up to $\sim10^{47}$ erg s$^{-1}$ for deep encounters.

Motivation & Objective

• To investigate how black hole spin and orbital inclination affect disc formation in tidal disruption events (TDEs) of stars on eccentric orbits.
• To determine the role of relativistic apsidal precession and Lense-Thirring nodal precession in stream self-intersection and circularisation.
• To assess the impact of radiative cooling on remnant disc structure and accretion dynamics in general relativistic TDE simulations.
• To quantify energy dissipation and accretion rates in TDEs with varying penetration factors, comparing adiabatic and isentropic cooling regimes.

Proposed method

• 3D general relativistic smoothed particle hydrodynamics (GRSPH) is employed in a fixed Kerr metric to model stellar tidal disruption by 10⁶M⊙ supermassive black holes.
• Stars of 1M⊙ are disrupted on initially eccentric orbits, both coplanar and inclined to the black hole equatorial plane.
• Relativistic effects such as apsidal precession and Lense-Thirring nodal precession are accurately captured via the Kerr metric in the GRSPH framework.
• Two cooling regimes are simulated: fully adiabatic (no cooling) and isentropic (instantaneous cooling), to bound the true physical behavior.
• Energy dissipation rates are calculated from shock formation, stream-stream collisions, and viscous heating, while accretion rates are tracked at r = 5Rg.
• Simulations are restricted to prograde, bound orbits, with no magnetic fields or stellar structure effects included.

Experimental results

Research questions

• RQ1 Does black hole spin-induced nodal precession prevent stream self-intersection and disc formation in TDEs with inclined stellar orbits?
• RQ2 How does radiative cooling influence the morphology and dynamics of the remnant accretion disc following tidal disruption?
• RQ3 What is the relationship between the penetration factor β and the energy dissipation rate in TDEs?
• RQ4 How do adiabatic and isentropic cooling approximations affect the structure and lifetime of the post-disruption remnant?
• RQ5 To what extent do relativistic precession effects (apsidal and nodal) govern the timescale of disc formation?

Key findings

• Nodal precession from black hole spin delays disc formation by only a short time, without preventing it, even for highly inclined orbits.
• Radiatively efficient cooling produces a narrow, rapidly precessing ring of material near pericentre, while no cooling leads to a thick, extended torus with aspect ratio ~1.
• The remnant disc precesses about the black hole spin axis due to Lense-Thirring effects, regardless of cooling regime.
• Energy dissipation rates reach ~10^45 erg s⁻¹ for disruptions at the tidal radius and up to ~10^47 erg s⁻¹ for deep encounters with high penetration factors.
• TDEs with β = 5 produce energy dissipation rates roughly two orders of magnitude higher than those with β = 1.
• Numerical viscosity drives accretion in the absence of magnetic fields, but this may overestimate accretion rates and underestimate disc lifetimes.