Super-Eddington Chimneys: On the Cooling Evolution of Tidal Disruption Event Envelopes

TL;DR

This study uses radiation-hydrodynamic simulations to explore how tidal disruption event envelopes cool and contract rapidly, forming dense rings that explain early UV/optical emissions without relying on viscosity or accretion.

Contribution

It demonstrates that cooling-induced contraction of TDE envelopes occurs on shorter timescales than photon diffusion, highlighting the importance of advective and wind-driven energy transport.

Findings

Envelope cooling enables rapid contraction and dense ring formation.

Cooling timescale weakly depends on optical depth, dominated by advection and winds.

Simulated luminosities and radii match early TDE UV/optical observations.

Abstract

The formation of a compact accretion disk following a tidal disruption event (TDE) requires that the shocked stellar debris cool efficiently as it settles toward the black hole. While recent simulations suggest that stream dissipation occurs rapidly, how the weakly bound debris subsequently loses its thermal energy to assemble a compact disk near the circularization radius remains uncertain. We investigate this cooling process using axisymmetric radiation-hydrodynamic simulations of quasi-hydrostatic 'TDE envelopes', initialized with the total mass, angular momentum, and binding energy expected from a complete stellar disruption. The envelopes, supported by radiation pressure on large scales and rotation near the circularization radius, evolve through a combination of radiative diffusion, turbulent mixing, and polar outflows. In our fiducial model, a quasi-steady state is achieved in which a polar outflow radiates and expels matter at several times the Eddington luminosity. This enables the envelope to cool and contract, forming a dense, rotationally supported ring near the circularization radius, but on a timescale roughly ten times shorter than the naive photon-diffusion timescale. Comparative models without radiation transport confirm that cooling, not purely adiabatic evolution, is essential to driving this rapid inflow. Nevertheless, across a range of envelope masses, the effective envelope cooling time scales only weakly with its optical depth, implying that advective and wind-driven energy transport dominate over diffusion. Our results demonstrate the cooling-induced contraction, even absent viscosity and associated black hole accretion, can produce luminosities and large photosphere radii consistent with early UV/optical TDE emission. However, more quantitative light-curve predictions must incorporate self-consistent formation and feeding of the envelope by fall-back accretion.