Accretion Disks

Accretion discs constitute an intermediate stage of a collapse process when matter has finite angular momentum, keeping it in a keplerian orbit where centrifugal force balances gravity. With the presence of accretion discs, however, mechanisms for removal of the angular momentum are required for further accretion onto the surface of the central object (proto-star, neutron-star or black hole).

Axel Brandenburg was at the forefront of a research effort which helped isolate one such a mechanism. Indeed, direct simulations of turbulence in these accretion discs helped prove the existence of hydromagnetic turbulence in the discs which gradually transported some angular momentum radially outwards in the disc, contributing to partially solving the angular momentum problem. In fact, these simulations gave an estimate of the magnitude of enhanced turbulent viscosity in discs leading to the release of extreme energies through turbulent viscous processes. These results were further strengthened by the work of Balbus & Hawley who pointed out a source for such a turbulence, namely, through magneto-rotational instability (nonlinear instabilities or convection were ruled out). Nordita staff was also involved in the first simulations showing the self-sustainance of dynamo-generated turbulence. This meant that large scale fields can be produced naturally. Such large scale fields in turn are of crucial importance for launching and collimating outflows from accretion discs.