Other
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...18110604k&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #106.04D; Bulletin of the American Astronomical Society, Vol. 24, p.1291
Other
Scientific paper
Approximately 3-10% of the observed QSOs exhibit broad absorption lines (BALs) associated with high ionization species such as C IV. A typical BAL has an absorption trough displaced to the blue side of the corresponding emission peak (from the broad emission line region). The absorption trough is usually asymmetric, having a sharp red edge, a diffuse blue tail, and a total width on the order of 0.1c. One explanation for BALs involves cool, dense gas clouds moving radially outward from the continuum source (with viewing angle explaining the non-BALQSOs). The main objection to this ``cloudy'' model is cloud stability. How do clouds, vulnerable to various fluid instabilities, remain cool and absorbing while being accelerated to high velocities? To explain the stability of accelerated clouds, we employ ablation to limit the growth of disruptive instabilities and prevent rapid heating of the cloud. For a dense cloud embedded in a supersonic flow, a bow shock will form upstream. The shocked wind gas, flowing around the periphery of the cloud, will ablate and limit the growth of Rayleigh-Taylor ``fingers''. In so doing, ablative mass loss effectively ``stabilizes'' the cloud, while simultaneously transporting heated cloud material into the shocked flow. Thus, the BAL shape naturally arises from this model: the sharp red edge represents the main cloud; the diffuse blue tail represents the faster moving ``ablata''; the displacement of the BAL trough corresponds to the acquired velocity of the cloud-ablata system. To study the above scenario, we have numerically simulated (using Eulerian hydrodynamics) a dense cloud embedded in a high speed wind. From axi- and slab-symmetric 2D calculations, we have verified the ablation mechanism and accelerated clouds to speeds (on average) of ~ 15%vwind. Further 2D results and a fully 3D calculation will be presented. Simulated absorption line profiles will be given for comparison with observations. Various other diagnostics, including a fractal analysis comparing the cloud surface area to cloud volume, will also be presented.
Christiansen Wayne A.
Knerr Jeffrey Matthew
Schiano V. R. A.
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