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).
A. 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.
Further reading:
Brandenburg, A., Nordlund, Å., Stein, R. F., & Torkelsson, U.: 1995, ``Dynamo-generated turbulence and large scale magnetic fields
in a Keplerian shear flow,'' Astrophys. J. 446, 741-754
(ADS)
Brandenburg, A.: 1998, ``Disc Turbulence and Viscosity,'' in Theory of Black Hole Accretion Discs, ed. M. A. Abramowicz, G. Björnsson & J. E. Pringle, Cambridge University Press, pp. 61-86
(ADS,
http://www.nordita.dk/~brandenb/papers/iceland.html)
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