The Generic NRAF Model
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5.
References
The Generic NRAF Model
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5.
References
In this paper, we have examined two global MHD simulations of a
magnetized nonradiative optically thin accretion flow onto a black
hole. Thermal conduction is not included, and the gas obeys a 
adiabatic equation of state. The initial state is a constant
angular momentum torus located at 100 Schwarzschild radii from the
hole. Such flow is unstable according to the generalized adiabatic
criteria of Balbus (1995), becomes fully turbulent, and in the process
transports angular momentum and energy outward and greatly enhanced
rates. Several distinctive properties characterize the resulting
flow.
Three principal flow structures are apparent: a hot Keplerian disk that extends down to the last marginally stable orbit, a coronal envelope, and a jet along the centrifugal barrier, or funnel wall.
Although the flow is nonradiative and hot, it remains centrifugally
supported. The density-weighted specific angular momentum distribution
is nearly Keplerian,
.
There are only small
sub-Keplerian departures from this. The near-Keplerian distribution
arises naturally from the vigorous MRI-induced turbulence, which
transports angular momentum outwards until radial pressure gradients
are minimal. No circumstances have been observed in any simulations
done to date where a significant non-Keplerian angular momentum
distribution (e.g. constant=l) could be sustained over a large
radial extent in the face of the MRI transport.
The only location where radial pressure support becomes somewhat important is in the innermost region of the flow where a hot thickened toroidal structure forms. This is constantly created and destroyed. Such a hot inner torus may be the origin of the submillimeter excess associated with Sgr A*.
The entropy and temperature increase inwards. In the simulation the entropy is increased by shock heating (artificial viscosity), and resistivity. Despite this entropy gradient, convection appears to play no role in the dynamics of the flow, whose turbulent properties are determined entirely by the MRI. Marginal stability to the classical Høiland criteria is not achieved, nor could it be achieved as a matter of principle (Hawley et al. 2001). Thus, although little of the mass that accretes at large radius makes it into the hole (in contrast to an ADAF), this is not a CDAF. Mass and energy are carried off by an outflow, more in keeping with the outline of the ADIOS model.
The flow is highly dynamic, inherently multidimensional, and time-variable on all scales. The flow reacts locally to the MRI, which, because it is a fast growing instability, changes on timescales shorter than the local orbital time. Note that even if there exists a time-averaged description of the global flow that is relatively time-steady, this need not correspond to an analytic global equilibrium.
Because these features follow directly from the self-consistent MHD dynamics, we believe that they will be generic to nonradiative accretion flows. Longer evolutions would go farther in establishing that the simulation results have lost memory of the initial conditions. Also, models with a greater radial domain would allow the inflow to proceed over several decades in distance and to liberate greater amounts of gravitational energy. Higher resolution is needed in the inner region to capture the details of the disk boundary and the flow past the marginally stable orbit.
Our data sets in principle readily lend themselves to observational modeling. A very preliminary application to Sgr A* relies on low accretion rates into the hole and emission from a hot, time-varying inner region. A detailed follow-up investigation on the radiative properties of our simulated flows is currently being pursued.
We acknowledge support under NSF grant AST-0070979, and NASA grant NAG5-9266. The simulations were carried out on the IBM Bluehorizon system of the San Diego Supercomputer Center. We thank Julian Krolik and James Stone for useful discussions related to this work.