question concerns the accretion timescale defining the growth of those seeds. The accretion rate is fixed by the mechanism of angular momentum transfer and depends on the gas viscosity mechanism. Presently there is no adequate physical theory able to describe the gas viscosity in the presence of turbulent flows or in the presence of magnetic fields. The angular momentum transfer is generally described by the formalism introduced almost forty five years ago by Shakura & Sunyaev (1973), in which the viscosity is due to subsonic turbulence and is parametrized by the relation $\eta = {\alpha}Hc_s$, where $\eta$ is the viscosity, $\alpha \leq 1$ is a free parameter of the theory, $c_s$ is the sound velocity and $H$ is the vertical scale of height of the disk. $H$ is supposed to be of the same order as the typical (isotropic) turbulence scale. However, disks based on such a formalism are, in general, thermally unstable as demonstrated long time ago by Piran (1978).

It is well known that self-gravitating disks may be also unstable but, in some cases, such an instability can be a source of turbulence in the flow (Duschl & Britsch 2006). Simulations of the gas inflow in the central 