ON THE ANGULAR-MOMENTUM TRANSPORT ASSOCIATED WITH CONVECTIVE EDDIES IN ACCRETION DISKS

Protostellar disks are intrinsically unstable against thermal convecti on in the direction normal to the plane of the disk. Because the origi n of effective disk viscosity is unknown it is of great theoretical in terest to investigate the effect of convection on the transport of ang ular momentum. To study this effect we present here results of direct two-dimensional simulations of compressible convection in axisymmetric accretion disks. In order to make the problem tractable an underlying viscosity (possibly due to shear instabilities and/or smaller scale c onvection) is used such that the calculations are performed for a Rayl eigh number 10 times the critical value for marginal stability. Under these conditions the results show that the convection cells cross the equatorial plane and extend over the entire vertical height. They have , for the chosen underlying viscosity, a radial extension comparable t o but somewhat less than the disk height. We show that the convection modifies both the heat and angular momentum transport. The modificatio n is such that in order to transmit a given outward angular momentum f lux coming from a stable region, the structure adjusts such that the m ean viscous flux is increased, this being counterbalanced by a negativ e advected flux. This suggests that the nonlinear viscous dissipation and mixing associated with large-scale axisymmetric convective motions leads to an inward flux of angular momentum. However, we point out th at this result may not apply to the convective motion when the underly ing viscosity is much smaller or when nonaxisymmetric modes are consid ered.