Exploring the origin of stars on bound and unbound orbits causing tidal disruption events

TL;DR

This paper develops an analytical model to understand the orbital origins of stars causing tidal disruption events, confirming most originate from the full loss-cone region and are marginally eccentric or hyperbolic.

Contribution

It introduces an analytical model based on the Fokker-Planck approach to describe the density and velocity distribution of stars involved in TDEs, validated by N-body simulations.

Findings

Most TDE stars originate from the full loss-cone region.

Stars causing TDEs are marginally eccentric or hyperbolic.

Analytical boundaries in eccentricity-$eta$ space are confirmed by N-body data.

Abstract

Tidal disruption events (TDEs) provide a clue to the properties of a central supermassive black hole (SMBH) and an accretion disk around it, and to the stellar density and velocity distributions in the nuclear star cluster surrounding the SMBH. Deviations of TDE light curves from the standard occurring at a parabolic encounter with the SMBH depends on whether the stellar orbit is hyperbolic or eccentric (Hayasaki et al. 2018) and the penetration factor ($\beta$, tidal disruption radius to orbital pericenter ratio). We study the orbital parameters of bound and unbound stars being tidally disrupted by comparison of direct $N$-body simulation data with an analytical model. Starting from the classical steady-state Fokker-Planck model of Cohn & Kulsrud (1978), we develop an analytical model of the number density distribution of those stars as a function of orbital eccentricity ($e$) and $\beta$. To do so fittings of the density and velocity distribution of the nuclear star cluster and of the energy distribution of tidally disrupted stars are required and obtained from $N$-body data. We confirm that most of the stars causing TDEs in a spherical nuclear star cluster originate from the full loss-cone region of phase space, derive analytical boundaries in eccentricity-$\beta$ space, and find them confirmed by $N$-body data. Since our limiting eccentricities are much smaller than critical eccentricities for full accretion or full escape of stellar debris, we conclude that those stars are only very marginally eccentric or hyperbolic, close to parabolic.