Quiescent accretion disk atmospheres
The physical process of viscous dissipation in accretion disks is
poorly understood. Accretion results from the transfer of angular
momentum outwards by an unknown viscous action between shearing
material. Progress in accretion theory has been made largely by
packing viscosity physics inside a dimensionless parameter (Shakura &
Sunyaev 1973). More sophisticated approaches which regard viscosity to
be the result of magnetic stresses, hydrodynamic turbulence and/or
tidal action have been encouraging but theorists have enormous freedom
to maneuver in an area which has been poorly constrained by
observation.
From an observational perspective, spectra contain potentially the
information to determine fundamental atmospheric properties of
quiescent atmospheres. However a broad range of physical conditions
occur within the atmosphere of any accretion disc where the cool outer
disc is a very different environment to the hot inner
region. Consequently the characteristics of emergent disc spectra are
strong functions of spatial location. Astrophysical discs cannot be
resolved in general because of their size and distance, and therefore
our problem is that classical observations provide only a global
picture of accretion disc behaviour.
Our solution to this problem combines sophisticated atmosphere models
with indirect imaging techniques and applies these to spectroscopic
observations of quiescent discs to determine the local physical
properties of their atmospheres, and resolve their spatial dependence.
The discipline of disc atmosphere building has now reached a
sophistication with which the physical state of discs can be
realistically modelled. Our models can include the effects of external
irradiation, vertical gravity gradients, finite optical depths, energy
dissipation, turbulence and supersonic velocity gradients. Our current
code has been used with notable success in probing the structure of
absorption curtains in accretion discs and to investigate optically
thick, steady-state atmospheres (Horne et al. 1994). A successful fit
to synthetic data is provided in the figure.
Horne K., Marsh T.R., Cheng F.H., Hubeny I., Lanz T., 1994, ApJ, 426, 294
Shakura N.I., Sunyaev R.A., 1973, A&A, 24, 337
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