Observations show that the central black hole in galaxies has a mass M only ∼ 10 of the stellar bulge mass. Thus whatever process grows the black hole also promotes star formation with far higher efficiency. We interpret this in terms of the generic tendency of AGN accretion discs to become self–gravitating outside some small radius Rsg ∼ 0.01− 0.1 pc from the black hole. We argue that mergers ...

Numerical simulations of turbulent, magnetized, differentially rotating flows driven by the magnetorotational instability are often used to calculate the effective values of alpha viscosity that is invoked in analytical models of accretion discs. In this paper we use various dynamical models of turbulent magnetohydrodynamic stresses as well as numerical simulations of shearing boxes to show tha...

Accretion discs are ubiquitous in astrophysics and in particular in accreting objects like cataclysmic variables, X-ray binaries or young stars. For many years magnetic ®elds have been included in models of such objects to be able to generate out ̄ows or jets or to explain accretion ofmatter on to the poles of the central objects. The interest in magnetic ®elds in accretion systems has been rece...

It has recently been noted that many discs around T Tauri stars appear to comprise only a few Jupiter masses of gas and dust. Using millimetre surveys of discs within six local star formation regions, we confirm this result, and find that only a few per cent of young stars have enough circumstellar material to build gas giant planets, in standard core accretion models. Since the frequency of ob...

We study the structure and dynamics of the gap created by a protoplanet in an accretion disc. The hydrodynamic equations for a flat, two-dimensional, non-selfgravitating protostellar accretion disc with an embedded, Jupiter sized protoplanet on a circular orbit are solved. To simulate possible accretion of mass onto the protoplanet we continually remove mass from the interior of the planet’s Ro...

Numerical simulations of turbulent, magnetized, differentially rotating flows driven by the magnetorotational instability are often used to calculate the effective values of alpha viscosity that is invoked in analytical models of accretion discs. In this paper we use various dynamical models of turbulent magnetohydrodynamic stresses, as well as numerical simulations of shearing boxes, to show t...