GRAINS IN IONIZED NEBULAE .2. 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 th at dust exists in the ionized regions of nebulae. In this paper, we co nsider the elements Sc, Ti, V, and Cr, all of which are strongly deple ted in the general interstellar medium. We construct simple three-leve l 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 sho uld be easily detectable, provided that their gas-phase abundances are solar. This suggests that these elements are strongly depleted in ion ized regions of these nebulae. We quantify these expectations by defin ing and comparing line ratios which are relatively insensitive to stel lar and nebular parameters with recently measured intensities of [V IV ], [Cr IV], and [Cr V] lines in NGC 7027. We encourage both further th eoretical and observational work on these ions.