Meridional circulation and reverse advection in hot thin accretion discs

TL;DR

This paper investigates the vertical flow patterns in hot thin accretion discs, revealing conditions under which midplane outflows and reverse heat advection can occur, challenging traditional inward-flow assumptions.

Contribution

It demonstrates that meridional circulation can cause midplane outflows and reverse heat advection in certain accretion disc regimes, highlighting the importance of vertical structure in disc dynamics.

Findings

Gas-pressure-dominated discs can have midplane outflows.

Standard radiation-pressure-dominated discs generally move inward at all heights.

Quasi-spherical pressure scaling can lead to broad midplane outflows.

Abstract

In standard accretion discs, outward angular momentum transfer by viscous forces is compensated by the inward motion of the accreting matter. However, the vertical structure of real accretion discs leads to meridional circulation with comparable amplitudes of poloidal velocities. Using thin-disc approximation, we consider different regimes of disc accretion with different vertical viscosity scalings. We show that, while gas-pressure-dominated discs can easily have a midplane outflow, standard thin radiation-pressure-dominated disc is normally moving inwards at all the heights. However, quasi-spherical scaling for pressure ($p\propto \varpi^{-5/2}$) leads to a midplane outflow for a very broad range of parameters. It particular, this may lead to a reversed, outward heat advection in geometrically thick discs when the temperature decreases rapidly enough with height. While the overall direction of heat advection depends on the unknown details of vertical structure and viscosity mechanisms, existence of the midplane counterflow in quasi-spherical flows is a robust result weakly dependent on the parameters and the assumptions of the model. Future models of thick radiatively inefficient flows should take meridional circulation into account.