Viscous Instability Triggered by Layered Accretion in Protoplanetary Disks

Viscous Instability Triggered by Layered Accretion in Protoplanetary Disks

Authors:

Hasegawa et al

Abstract:

Layered accretion is one of the inevitable ingredients in protoplanetary disks when disk turbulence is excited by magnetorotational instabilities (MRIs). In the accretion, disk surfaces where MRIs fully operate have a high value of disk accretion rate (M˙), while the disk midplane where MRIs are generally quenched ends up with a low value of M˙. Significant progress on understanding MRIs has recently been made by a number of dedicated MHD simulations, which requires improvement of the classical treatment of α in 1D disk models. To this end, we obtain a new expression of α by utilizing an empirical formula that is derived from recent MHD simulations of stratified disks with Ohmic diffusion. It is interesting that this new formulation can be regarded as a general extension of the classical α. Armed with the new α, we perform a linear stability analysis of protoplanetary disks that undergo layered accretion, and find that a viscous instability can occur around the outer edge of dead zones. Disks become stable in using the classical α. We identify that the difference arises from Σ−dependence of M˙; whereas Σ is uniquely determined for a given value of M˙ in the classical approach, the new approach leads to M˙ that is a multi-valued function of Σ. We confirm our finding both by exploring a parameter space as well as by performing the 1D, viscous evolution of disks. We finally discuss other non-ideal MHD effects that are not included in our analysis, but may affect our results.