Radial Stresses and Energy Transport in Accretion Disks

TL;DR

This paper explores how radial energy transport and boundary conditions in accretion disks influence their accretion rates and variability, highlighting the importance of energy modulation in the inner regions for the outer disk dynamics.

Contribution

It introduces the idea that energy supply modulations at the inner disk can rapidly affect the outer disk's accretion rate and variability, emphasizing the role of large-scale radial energy transport.

Findings

Inner boundary conditions influence outer disk accretion rates.

Energy modulations can cause rapid fluctuations in accretion.

Radial energy transport critically impacts disk evolution.

Abstract

Early in the study of viscous accretion disks it was realized that energy transfers from distant sources must be important, not least because the flow at the disk midplane in the bulk of the disk is likely outwards, out of the gravitational potential well. If the source of the viscosity is powered by accretion, such as in the case of the magneto-rotational instability, such distant energy sources must lie in the innermost regions of the disk, where accretion occurs even at the midplane. We argue here that modulations in this energy supply can alter the accretion rate on dynamical, rather than far longer viscous, time scales. This means that both the steady state value of and fluctuations in the inner disk's accretion rate, depending on the details of the inner boundary condition and occurring on the inner disk's rapid evolution time, can affect the outer disk. This is particularly interesting because observations have shown that disk accretion is not steady (e.g.~EX Lupi type objects). We also note that the power supplied to shearing boxes is set by the boxes themselves rather than the physical energy fluxes in a global disk. That is, their saturated magnetic field is not subject to the full set of energy constraints present in an actual disk. Our analysis suggests that large scale radial transport of energy has a critical impact on the evolution and variability of accretion disks.