Metal-Ion Absorption in Conductively Evaporating Clouds
  We present computations of the ionization structure and metal-absorption
properties of thermally conductive interface layers that surround evaporating
warm spherical clouds, embedded in a hot medium. We rely on the analytical
formalism of Dalton & Balbus to calculate the temperature profile in the
evaporating gas, and explicitly solve the time-dependent ionization equations
for H, He, C, N, O, Si, and S in the interface. We include photoionization by
an external field. We estimate how departures from equilibrium ionization
affect the resonance-line cooling efficiencies in the evaporating gas, and
determine the conditions for which radiative losses may be neglected in the
solution for the evaporation dynamics and temperature profile. Our results
indicate that non-equilibrium cooling significantly increases the value of the
saturation parameter at which radiative losses begin to affect the flow
dynamics. As applications we calculate the ion fractions and projected column
densities arising in the evaporating layers surrounding dwarf-galaxy-scale
objects that are also photoionized by metagalactic radiation. We compare our
results to the UV metal-absorption column densities observed in local
highly-ionized metal-absorbers, located in the Galactic corona or intergalactic
medium. Conductive interfaces significantly enhance the formation of high-ions
such as C^3+, N^4+, and O^5+ relative to purely photoionized clouds, especially
for clouds embedded in a high-pressure corona. However, the enhanced columns
are still too low to account for the O VI columns (~1e14 cm^-2) observed in the
local high-velocity absorbers. We find that O VI columns larger than ~1e13
cm^-2 cannot be produced in evaporating clouds. Our results do support the
conclusion of Savage & Lehner, that absorption due to evaporating O VI likely
occurs in the local interstellar medium, with characteristic columns of ~1e13
cm^-2.
