Does the circularization radius exist or not for low angular momentum accretion?

TL;DR

This study uses numerical simulations to investigate whether a distinct circularization radius exists in low angular momentum hot accretion flows, finding it does not in steady states but appears in transient events.

Contribution

The paper demonstrates that the classical circularization radius does not exist in steady hot accretion flows, but a similar 'turning point' appears in transient accretion scenarios.

Findings

No circularization radius in steady accretion flows.

A 'turning point' exists in transient accretion events.

Angular momentum profile is smooth in steady flows.

Abstract

If the specific angular momentum of accretion gas at large radius is small compared to the local Keplerian value, one usually believes that there exists a "circularization radius" beyond which the angular momentum of accretion flow is almost a constant while within which a disk is formed and the angular momentum roughly follows the Keplerian distribution. In this paper, we perform numerical simulations to study whether the picture above is correct in the context of hot accretion flow. We find that for a steady accretion flow, the "circularization radius" does not exist and the angular momentum profile will be smooth throughout the flow. However, for transient accretion systems, such as the tidal disruption of a star by a black hole, a "turning point" should exist in the radial profile of the angular momentum, which is conceptually similar to the "circularization radius". At this radius, the viscous timescale equals the life time of the accretion event. The specific angular momentum is close to Keplerian within this radius, while beyond this radius the angular momentum is roughly constant.