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Paper: Multi-phase High-Velocity Clouds toward HE 0226-4110 and PG 0953+414
Volume: 331, Extra-Planar Gas
Page: 19
Authors: Fox, A.J.; Savage, B.D.; Wakker, B.P.; Tripp, T.M.; Sembach, K.R.
Abstract: O VI absorption in high-velocity clouds has been detected with the FUSE satellite in many directions across the sky. We discuss the ionization conditions and kinematics of two high positive-velocity absorption systems toward HE 0226−4110 and PG 0953+414, for which high-quality STIS and FUSE data are available. In these sight-lines, the high-velocity O VI is accompanied by absorption in several neutral and weakly ionized species over the same velocity range, vLSR ≈ 100-200 km s−1. We apply CLOUDY modeling to determine whether photoionization is a feasible ionization mechanism in these clouds, assuming that they are bathed in the extragalactic radiation field. Photoionization can account for the observed column densities of C II, C III, Si II, and Si III if the clouds are several kpc in depth, but cannot explain the columns of C IV or O VI. Analysis of the absorption kinematics and high-ion column density ratios leads us to conclude that the highly ionized species originate in conductive interfaces at the surface of warm HVCs, which are embedded in a hot external medium. This hot medium is probably the highly extended Galactic corona that has been invoked to explain X-ray observations of O VII and O VIII absorption.
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