Post-periapsis pancakes: sustenance for self-gravity in tidal disruption events

TL;DR

This paper investigates how post-periapsis caustics in tidal disruption events cause density fluctuations that can lead to stream fragmentation, influencing the observable features of such cosmic phenomena.

Contribution

It introduces the concept of a post-periapsis caustic, or 'pancake', and demonstrates its role in destabilizing and fragmenting debris streams in tidal disruption events.

Findings

Post-periapsis caustic occurs near the tidal radius.

Density perturbations can lead to stream fragmentation.

Fragmentation depends on the equation of state of the debris.

Abstract

A tidal disruption event, which occurs when a star is destroyed by the gravitational field of a supermassive black hole, produces a stream of debris, the evolution of which ultimately determines the observational properties of the event. Here we show that a post-periapsis caustic -- a location where the locus of gas parcels comprising the stream would collapse into a two-dimensional surface if they evolved solely in the gravitational field of the hole -- occurs when the pericenter distance of the star is on the order of the tidal radius of the hole. It is demonstrated that this "pancake" induces significant density perturbations in the debris stream, and, for stiffer equations of state (adiabatic index $\gamma \gtrsim 5/3$), these fluctuations are sufficient to gravitationally destabilize the stream, resulting in its fragmentation into bound clumps. The results of our findings are discussed in the context of the observational properties of tidal disruption events.