What causes the fragmentation of debris streams in TDEs?

Abstract

A tidal disruption event (TDE) occurs when a star passes too close to a supermassive black hole and gets torn apart by its gravitational tidal field. After the disruption, the stellar debris form an expanding gaseous stream. The morphology and evolution of this stream are particularly interesting as it ultimately determines the observational properties of the event itself. In this work, we perform 3D hydrodynamical simulations of the TDE of a star modelled as a polytropic sphere of index γ = 5/3 and study the gravitational stability of the resulting gas stream. We provide an analytical solution for the evolution of the stream in the bound, unbound, and marginally bound cases, which allows us to describe the stream properties and analyse the time-scales of the physical processes involved, applying a formalism developed in star formation context. Our results are that, when fragmentation occurs, it is fuelled by the failure of pressure in supporting the gas against its self-gravity. We also show that a stability criterion that includes also the stream gas pressure proves to be far more accurate than one that only considers the black hole tidal forces, giving analytical predictions of the time evolution of the various forces associated with the stream. Our results point out that fragmentation occurs on time-scales longer compared with the observational windows of these events and is thus not expected to give rise to significant observational features.

The dynamics of debris streams from tidal disruption events: exact solutions, critical stream density, and hydrogen recombination

“…Most recently Bonnerot et al (2022) used a Lagrangian technique to model the evolution of the debris stream from the initial destruction of the star to the return of the most bound debris, using the frozen-in approximation for the initial conditions (Kochanek 1994 used the affine-star model of Carter & Luminet 1983 to establish the initial conditions to solve his equations of motion). Coughlin & Nixon (2015), who numerically simulated the debris stream evolution from the canonical TDE with the SPH code - (Price et al 2018), found that the stream would fragment under its own self-gravity into localized knots (see also Hayasaki et al 2020;Sacchi et al 2020), which was not predicted by earlier models (Guillochon et al 2014b argued that the combination of radiative cooling and Kelvin-Helmholtz instability could result in the formation of a clump, and did not account for the stream self-gravity in their simulations). Using their Eulerian and semi-analytical model described above, Coughlin et al (2016b) argued that the ability of the stream to fragment under its own self-gravity is critically related to its equation of state.…”