High luminosity and short time-scales of variability are typical
for disks which collect matter from the surroundings to accrete
the gas and dust onto a central object. The efficiency of that
transport requires turbulence in the disk. Searches for
instabilities in such accretion disks revealed the magneto-rotational
instability as a powerful mechanism in a wide range of objects
(protostellar disks, cataclysmic binaries, active galactic nuclei):
A weak magnetic fields threads a differentially rotating disk
and excites an instability.
The global simulation of such
disk configurations on a computer have
recently become a challenge for modern computational astrophysics.
The above picture shows five horizontal slices through a simulated
accretion disk at five different height levels. The middle slice
is the equatorial plane. The colour shading represents the
density.
The tubulent flows in the simulated disks turn out to provide
powerful transport as required to match the observed phe
nomena.
Additional generation of strong, large-scale magnetic fields
may further illuminate our understanding of what is called
dynamo action and the launch of collimated outflows
(jets) from the disk as observed in protostellar disks and active
galactic nuclei.
( credit: R. Arlt, G. Rüdiger )
