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Henley, David B.,Kwak, Kyujin,Shelton, Robin L. IOP Publishing 2012 The Astrophysical journal Vol.753 No.1
<P>In order to determine if the material ablated from high-velocity clouds (HVCs) is a significant source of low-velocity high ions (C IV, NV, and OVI) such as those found in the Galactic halo, we simulate the hydrodynamics of the gas and the time-dependent ionization evolution of its carbon, nitrogen, and oxygen ions. Our suite of simulations examines the ablation of warm material from clouds of various sizes, densities, and velocities as they pass through the hot Galactic halo. The ablated material mixes with the environmental gas, producing an intermediate-temperature mixture that is rich in high ions and that slows to the speed of the surrounding gas. We find that the slow mixed material is a significant source of the low-velocity OVI that is observed in the halo, as it can account for at least similar to 1/3 of the observed OVI column density. Hence, any complete model of the high ions in the halo should include the contribution to the OVI from ablated HVC material. However, such material is unlikely to be a major source of the observed CIV, presumably because the observed C IV is affected by photoionization, which our models do not include. We discuss a composite model that includes contributions from HVCs, supernova remnants, a cooling Galactic fountain, and photoionization by an external radiation field. By design, this model matches the observed OVI column density. This model can also account for most or all of the observed C IV, but only half of the observed NV.</P>