Metal-Ion Absorption in Conductively Evaporating Clouds

TL;DR

This paper models the ionization and metal-absorption properties of conductive evaporating clouds in hot media, highlighting non-equilibrium effects and comparing predictions with observations of high-ion absorption in galactic and intergalactic environments.

Contribution

It introduces a detailed calculation of ionization structures in evaporating clouds, including non-equilibrium effects and external photoionization, to better understand observed metal absorption lines.

Findings

Non-equilibrium cooling increases the saturation parameter for radiative effects.

Conductive interfaces enhance high-ion formation compared to pure photoionization.

O VI columns in evaporating clouds are limited to ~1e13 cm^-2, insufficient to explain some observations.

Abstract

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.