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J. E. Bjorkman, A. C. Carciofi (Univ. Toledo)
It is now commonly believed that many kinds of hot stars are surrounded by gaseous Keplerian disks. Examples include the accretion disks of Herbig Ae/Be stars and the outflowing decretion disks of classical Be stars. To study these ionized disks, we have developed a fully 3-D Monte Carlo radiation transfer code that solves simultaneously the thermal equilibrium and the statistical equilibrium equations, thereby providing the NLTE electron temperature, ionization fraction, and hydrogen level populations throughout the circumstellar envelope. For our disk model, we adopt a steady-state viscous accretion/decretion disk. Since the vertical distribution of the disk material depends on the gas temperature, we also solve the coupled problem of radiative equilibrium and hydrostatic equilibrium of the disk. This procedure results in a fully self-consistent solution for the temperature, density, and ionization structure of the disk.
We find that, in the inner regions of the disk, the temperature falls rapidly with radius (similar to the dusty disks of low mass YSOs); however, beyond the location where the disk becomes optically thin, the temperature rises back up (to about 80% of the stellar temperature) and becomes isothermal at large radii. This implies that there is little increase of the disk scale height at small radii, but at larger distances, the disk begins to flare quite dramatically. Finally, if the disk is dense enough, there is a significant amount of neutral hydrogen that shields the disk interior from ionizing radiation, allowing anomalously low ionization states to exist quite close to the hot star.
This work was supported by NSF Grants AST-9819928 and AST-0307686.
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Bulletin of the American Astronomical Society, 36 #2
© YEAR. The American Astronomical Soceity.