Thermal equilibrium curves and turbulent mixing in Keplerian accretion disks

We consider vertical heat transport in Keplerian accretion disks, including the effects of radiation, convection, and turbulent mixing driven by the B albus-Hawley instability, in astronomical systems ranging from dwarf novae (DNe) and soft X-ray transients (SXTs) to active galactic nuclei (AGNs). In order to account for the interplay between convective and turbulent energy transport in a shearing environment, we propose a modified, anisotropic fo rm of mixing-length theory that includes radiative and turbulent damping. W e also include turbulent heat transport, which acts everywhere within disks , regardless of whether or not they are stably stratified and can move entr opy in either direction. We have generated a series of vertical structure m odels and thermal equilibrium curves using the scaling law for the viscosit y parameter alpha suggested by the exponential decay of the X-ray luminosit y in SXTs. We have also included equilibrium curves for DNe using an cc tha t is constant down to a small magnetic Reynolds number (similar to 10(4)). Our models indicate that weak convection is usually eliminated by turbulent radial mixing, even when mixing length estimates of convective vertical he at transport are much larger than turbulent heat transport. The substitutio n of turbulent heat transport for convection is more important on the unsta ble branches of thermal equilibrium S curves when alpha is larger. The low temperature turnover points Sigma(max) on the equilibrium S curves are sign ificantly reduced by turbulent mixing in DN and SXT disks. However, in AGN disks the standard mixing-length theory for convection is still a useful ap proximation when we use the scaling law for alpha since these disks are ver y thin at the relevant radii. In accordance with previous work, we find tha t constant alpha models give almost vertical S curves in the Sigma-T plane and consequently imply very slow, possibly oscillating, cooling waves.