Grains in ionized nebulae. II. Heavy-element depletion

The presence of grains in gaseous nebulae can have significant effects on the thermal balance and radiative line transfer in these objects. The depletion of condensable elements onto grains provides evidence that dust exists in the ionized regions of nebulae. In this paper, we consider the elements Sc, Ti, V, and Cr, all of which are strongly depleted in the general interstellar medium. We construct simple three-level atoms for several ions of these elements, and incorporate them into our photoionization code CLOUDY. For both a model planetary nebula and a model H II region, we find that several lines of these elements should be easily detectable, provided that their gas-phase abundances are solar. This suggests that these elements are strongly depleted in ionized regions of these nebulae. We quantify these expectations by defining and comparing line ratios which are relatively insensitive to stellar and nebular parameters with recently measured intensities of [V IV], [Cr IV], and [Cr V] lines in NGC 7027. We encourage both further theoretical and observational work on these ions.