Heating or Cooling: Study of Advective Heat Transport in the Inflow and the Outflow of Optically Thin Advection-dominated Accretion Flows

TL;DR

This paper analyzes the role of advective heat transport in optically thin ADAFs with outflows, revealing how radial and latitudinal advection contribute to heating or cooling, and emphasizing the importance of latitudinal advection for energy balance.

Contribution

It provides a self-similar solution framework showing how advective heat transport varies in inflows and outflows of ADAFs, highlighting the necessity of latitudinal advection for energy conservation.

Findings

Radial advection acts as heating in inflow and cooling in outflow for certain density profiles.

Latitudinal advection is crucial for maintaining energy balance in ADAFs.

Overall, advection functions as a cooling mechanism that offsets viscous heating.

Abstract

Advection is believed to be the dominant cooling mechanism in optically thin advection-dominated accretion flows (ADAF's). When outflow is considered, however, the first impression is that advection should be of opposite sign in the inflow and the outflow, due to the opposite direction of radial motion. Then how is the energy balance achieved simultaneously? We investigate the problem in this paper, analysing the profiles of different components of advection with self-similar solutions of ADAF's in spherical coordinates ($r\theta\phi$). We find that for $n < 3\gamma/2-1$, where $n$ is the density index in $\rho \propto r^{-n}$ and $\gamma$ is the heat capacity ratio, the radial advection is a heating mechanism in the inflow and a cooling mechanism in the outflow. It becomes 0 for $n = 3\gamma/2-1$, and turns to a cooling mechanism in the inflow and a heating mechanism in the outflow for $n > 3\gamma/2-1$. The energy conservation is only achieved when the latitudinal ($\theta$-direction) advection is considered, which takes an appropriate value to maintain energy balance, so that the overall effect of advection, no matter the parameter choices, is always a cooling mechanism that cancels out the viscous heating everywhere. For the extreme case of $n=3/2$, latitudinal motion stops, viscous heating is balanced solely by radial advection, and no outflow is developed.