249 research outputs found
Turbulent Stresses in Local Simulations of Radiation-Dominated Accretion Disks, and the Possibility of the LIghtman-Eardley Instability
We present the results of a series of radiation-MHD simulations of a local
patch of an accretion disk, with fixed vertical gravity profile but with
different surface mass densities and a broad range of radiation to gas pressure
ratios. Each simulation achieves a thermal equilibrium that lasts for many
cooling times. After averaging over times long compared to a cooling time, we
find that the vertically integrated stress is approximately proportional to the
vertically-averaged total thermal (gas plus radiation) pressure. We map
out--for the first time on the basis of explicit physics--the thermal
equilibrium relation between stress and surface density: the stress decreases
(increases) with increasing surface mass density when the simulation is
radiation (gas) pressure dominated. The dependence of stress on surface mass
density in the radiation pressure dominated regime suggests the possibility of
a Lightman-Eardley inflow instability, but global simulations or shearing box
simulations with much wider radial boxes will be necessary to confirm this and
determine its nonlinear behavior.Comment: accepted for publication in The Astrophysical Journa
Vertical Structure of Gas Pressure-Dominated Accretion Disks with Local Dissipation of Turbulence and Radiative Transport
(shortened) We calculate the vertical structure of a local patch of an
accretion disk in which heating by dissipation of MRI-driven MHD turbulence is
balanced by radiative cooling. Heating, radiative transport, and cooling are
computed self-consistently with the structure by solving the equations of
radiation MHD in the shearing-box approximation. Using a fully 3-d and
energy-conserving code, we compute the structure of this disk segment over a
span of more than five cooling times. After a brief relaxation period, a
statistically steady-state develops. Measuring height above the midplane in
units of the scale-height H predicted by a Shakura-Sunyaev model, we find that
the disk atmosphere stretches upward, with the photosphere rising to about 7H,
in contrast to the approximately 3H predicted by conventional analytic models.
This more extended structure, as well as fluctuations in the height of the
photosphere, may lead to departures from Planckian form in the emergent
spectra. Dissipation is distributed across the region within roughly 3H of the
midplane, but is very weak at greater altitudes. Because fluctuations in the
dissipation are particularly strong away from the midplane, the emergent
radiation flux can track dissipation fluctuations with a lag that is only
0.1--0.2 times the mean cooling time of the disk. Long timescale asymmetries in
the dissipation distribution can also cause significant asymmetry in the flux
emerging from the top and bottom surfaces of the disk. Radiative diffusion
dominates Poynting flux in the vertical energy flow throughout the disk.Comment: accepted by Ap
Thermodynamics of an Accretion Disk Annulus with Comparable Radiation and Gas Pressure
We explore the thermodynamic and global structural properties of a local
patch of an accretion disk whose parameters were chosen so that radiation
pressure and gas pressure would be comparable in magnitude. Heating, radiative
transport, and cooling are computed self-consistently with the structure by
solving the equations of radiation MHD in the shearing-box approximation. Using
a fully 3-d and energy-conserving code, we compute the structure and energy
balance of this disk segment over a span of more than forty cooling times. As
is also true when gas pressure dominates, the disk's upper atmosphere is
magnetically-supported. However, unlike the gas-dominated case, no steady-state
is reached; instead, the total (i.e., radiation plus gas) energy content
fluctuates by factors of 3--4 over timescales of several tens of orbits, with
no secular trend. Because the radiation pressure varies much more than the gas
pressure, the ratio of radiation pressure to gas pressure varies over the
approximate range 0.5--2. The volume-integrated dissipation rate generally
increases with increasing total energy, but the mean trend is somewhat slower
than linear, and the instantaneous dissipation rate is often a factor of two
larger or smaller than the mean for that total energy level. Locally, the
dissipation rate per unit volume scales approximately in proportion to the
current density; the time-average dissipation rate per unit mass is
proportional to m^{-1/2}, where m is the horizontally-averaged mass column
density to the nearer of the top or bottom surface. As in our earlier study of
a gas-dominated shearing-box, we find that energy transport is completely
dominated by radiative diffusion, with Poynting flux carrying less than 1% of
the energy lost from the box.Comment: ApJ, in pres
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