1,362 research outputs found
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
Continuum Spectra of Quasar Accretion Disk Models
We have calculated the spectrum and polarization of a standard thin accretion
disk with parameters appropriate for a bright quasar. This model improves upon
previous work by including ultraviolet metal line opacities, assumed for now to
be in LTE. Though not yet fully self-consistent, our calculations demonstrate
that metal lines can change the spectral slope, reduce the polarization, and
reduce the Lyman edge feature in accretion disk spectra. Some observational
differences between quasar spectra and accretion disk models might be
reconciled with the inclusion of metal lines.Comment: 4 pages, 3 figures, to appear in "Accretion Processes in
Astrophysical Systems: Some Like it Hot," proceedings of the 8th Annual
October Astrophysics Conference in Marylan
Local Radiation MHD Instabilities in Magnetically Stratified Media
We study local radiation magnetohydrodynamic instabilities in static,
optically thick, vertically stratified media with constant flux mean opacity.
We include the effects of vertical gradients in a horizontal background
magnetic field. Assuming rapid radiative diffusion, we use the zero gas
pressure limit as an entry point for investigating the coupling between the
photon bubble instability and the Parker instability. Apart from factors that
depend on wavenumber orientation, the Parker instability exists for wavelengths
longer than a characteristic wavelength lambda_{tran}, while photon bubbles
exist for wavelengths shorter than lambda_{tran}. The growth rate in the Parker
regime is independent of the orientation of the horizontal component of the
wavenumber when radiative diffusion is rapid, but the range of Parker-like
wavenumbers is extended if there exists strong horizontal shear between field
lines (i.e. horizontal wavenumber perpendicular to the magnetic field). Finite
gas pressure introduces an additional short wavelength limit to the Parker-like
behavior, and also limits the growth rate of the photon bubble instability to a
constant value at short wavelengths. We also consider the effects of
differential rotation with accretion disk applications in mind. Our results may
explain why photon bubbles have not yet been observed in recent stratified
shearing box accretion disk simulations. Photon bubbles may physically exist in
simulations with high radiation to gas pressure ratios, but higher spatial
resolution will be needed to resolve the asymptotically growing unstable
wavelengths.Comment: The Astrophysical Journal, in pres
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