Properties of gravitoturbulent accretion disks

Abstract

We explore the properties of cold gravitoturbulent accretion disks - non-fragmenting disks hovering on the verge of gravitational instability - using a realistic prescription for the effective viscosity caused by gravitational torques. This prescription is based on a direct relationship between the angular momentum transport in a thin accretion disk and the disk cooling in a steady state. Assuming that opacity is dominated by dust we are able to self-consistently derive disk properties for a given $\dot M$ assuming marginal gravitational stability. We also allow external irradiation of the disk and account for a non-zero background viscosity which can be due to the MRI. Spatial transitions between different co-existing disk states (e.g. between irradiated and self-luminous or between gravitoturbulent and viscous) are described and the location of the boundary at which disk must fragment is determined in a variety of situations. We demonstrate in particular that at low enough $\dot M$ external irradiation stabilizes gravitoturbulent disk against fragmentation all the way to infinity thus providing means of steady mass transport to the central object. Implications of our results for the possibility of planet formation by gravitational instability in protoplanetary disks and star formation in the Galactic Center and for the problem of feeding supermassive black holes in galactic nuclei are discussed.