The persistence of pancakes and the revival of self-gravity in tidal disruption events

TL;DR

This paper demonstrates that in tidal disruption events (TDEs), a persistent in-plane pancake forms, reviving the role of self-gravity in debris streams, which can lead to gravitational instability and affects the evolution of the disrupted stellar material.

Contribution

It generalizes the formation of the in-plane pancake to all fully disrupted TDEs and shows how it enhances self-gravity effects in debris streams.

Findings

The in-plane pancake forms outside the tidal disruption radius.

Gas compression near the caustic is mildly supersonic, leading to density increases.

Self-gravity remains significant in debris streams post-pericenter.

Abstract

The destruction of a star by the tides of a supermassive black hole (SMBH) powers a bright accretion flare, and the theoretical modeling of such tidal disruption events (TDEs) can provide a direct means of inferring SMBH properties from observations. Previously it has been shown that TDEs with $\beta = r_{\rm t}/r_{\rm p} = 1$, where $r_{\rm t}$ is the tidal disruption radius and $r_{\rm p}$ is the pericenter distance of the star, form an in-plane caustic, or ``pancake,'' where the tidally disrupted debris is compressed into a one-dimensional line within the orbital plane of the star. Here we show that this result applies generally to all TDEs for which the star is fully disrupted, i.e., that satisfy $\beta \gtrsim 1$. We show that the location of this caustic is always outside of the tidal disruption radius of the star and the compression of the gas near the caustic is at most mildly supersonic, which results in an adiabatic increase in the gas density above the tidal density of the black hole. As such, this in-plane pancake revitalizes the influence of self-gravity even for large $\beta$, in agreement with recent simulations. This finding suggests that for all TDEs in which the star is fully disrupted, self-gravity is revived post-pericenter, keeps the stream of debris narrowly confined in its transverse directions, and renders the debris prone to gravitational instability.