Highly Ionized Interstellar Atoms—Heated, Cooled, or Mixed?

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Abstract
Recent observations with the Goddard High Resolution Spectrograph on the Hubble Space Telescope, combined with Copernicus results, make possible a comparison between C+3 and O+5 interstellar column densities in both the halo and the disk of our Galaxy. The ratio N(C+3)/N(O+5) for six lines of sight in the disk is about an order of magnitude less than for five corresponding values in the halo. In the disk, the values of this ratio are in good general agreement with a variety of different models for conductive heating at an interface between hot and cool gas. As a working hypothesis we assume that this process is the dominant one for producing the observed highly ionized species in the disk. The much larger ratios for the halo lie between the ranges predicted by two different idealized models—radiative cooling of hot infalling gas and turbulent mixing of hot gas with the cool clouds past which it flows. Since both these processes should occur when hot gas flows past H I clouds in the halo, we assume tentatively that these two processes may be jointly responsible for much of the observed high ionization of halo atoms.