Accretion disks : viscous or turbulent ?

Séminaire IPAG de Sébastien Fromang (CEA), jeudi 22 septembre 2011 à 11h00, IPAG seminar room

In accretion disks, outward angular momentum is believed to result from the action of MHD turbulence powered by the magnetorotational instability. Because of the complex nature of the flow that results, the effects of the turbulence are often modeled by enhanced transport coefficients. This approach has led for example to the famous alpha disk model, which has been used in a large number of applications. In some cases, the validity of such a modeling is questionable. In this talk, I will examine two problems relevant for protoplanetary disks for which this is the case. The first is to determine whether or not turbulent disk exerts a corotation torque on an embedded newly formed planet, as is the case of a viscous disk. The second question is that of the existence of a large scale mean flow in protoplanetary disks. Such a mean flow, called the meridional circulation, is indeed a prediction of viscous disk theory. In both situations, the goal is to determine whether or not the effect of the turbulence can be reduced to the action of an effective viscosity. This will be done through a detailed comparison between the predictions of a viscous model and the results of numerical simulations of protoplanetary disks in which the turbulent flow structure is computed from first principles.