Dr. JIAO Chengliang from the Yunnan Observatories of the Chinese Academy of Sciences has investigated the energy conservation in the inflow and the outflow of optically thin advection-dominated accretion flows. Results were published online in The Astrophysical Journal.

Optically thin advection-dominated accretion flows (ADAFs) are widely used to explain observations of the galactic center, low-luminosity active galactic nuclei and the low-hard state of black hole X-ray binaries. In classical model of ADAFs, cooling is believed to be dominated by advective heat transport (usually referred to as "advective cooling") due to the low radiative efficiency. However, the physical nature of advective heat transport is the net difference in entropy between the incoming and outgoing material for a specific region in the rest frame, so it could be cooling (losing entropy) or heating (gaining entropy) equally.

As theoretical works, observations and numerical simulations have all suggested that strong outflow exist in ADAFs, a natural conjecture is that advective heat transport should be of opposite sign in the inflow and the outflow where the radial motion is in opposite direction. Then how is the energy conservation maintained in both the inflow and the outflow simultaneously?

Based on direction (radial and latitudinal) and mechanism (advection of internal energy and pressure work), Dr. JIAO decomposed the advective heat transport, and examined how each component works in the inflow and outflow.

He found that in most cases, radial advection is actually a heating mechanism in the inflow, contrary to the popular interpretation of "advective cooling", since radial pressure work dominates over the radial advection of internal energy. The steady state can thus only be achieved when considering latitudinal advection, which carries entropy from the inflow to the outflow where it is cooled.

This work confirms and quantifies the perception that outflow (or wind) carries away entropy thus cooling the inflow.

Components of advective heat transport as functions of θ. Positive values represent cooling, and negative values heating. The subscripts r and θ represent radial and latitudinal direction, while 1 and 2 represent advected internal energy and pressure work, respectively. (Image by JIAO Chengliang)