  In order to determine if the material ablated from high-velocity clouds
(HVCs) is a significant source of low-velocity high ions (C IV, N V, and O VI)
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 O VI that is observed in
the halo, as it can account for at least ~1/3 of the observed O VI column
density. Hence, any complete model of the high ions in the halo should include
the contribution to the O VI from ablated HVC material. However, such material
is unlikely to be a major source of the observed C IV, 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 O VI column
density. This model can also account for most or all of the observed C IV, but
only half of the observed N V.
