The gravo-magneto disc instability with a viscous dead zone

Abstract

We consider the evolution of accretion discs that contain some turbulence within a disc dead zone, a region about the disc midplane of a disc that is not sufficiently ionised for the magneto-rotational instability (MRI) to drive turbulence. In particular, we determine whether additional sources of turbulence within a dead zone are capable of suppressing gravo-magneto (GM) disc outbursts that arise from a rapid transition from gravitationally produced to MRI produced turbulence. With viscous $\alpha$ disc models we consider two mechanisms that may drive turbulence within the dead zone. First, we examine a constant $\alpha$ parameter within the dead zone. This may be applicable to hydrodynamical instability, such as baroclinic instability, where the turbulence level is independent of the MRI active surface layer properties. In this case, we find that the disc will not become stable to the outbursts unless the dead zone turbulent viscosity is comparable to that in the MRI active surface layers. Under such conditions, the accretion rate through the dead zone must be larger than that through the MRI active layers. In a second model, we assume that the accretion flow though the dead zone is a constant fraction (less than unity) of that through the active layers. This may be applicable to turbulence driven by hydrodynamic waves that are excited by the MRI active surface layers. We find that the instability is hardly affected by the viscous dead zone. In both cases however, we find that the triggering of the MRI during the outburst may be due to the heating from the viscosity in the dead zone, rather than self-gravity. Thus, neither mechanism for generating turbulence within the dead zone can significantly stabilise a disc or the resulting outburst behaviour.