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