GRAINS IN IONIZED NEBULAE - SPECTRAL-LINE DIAGNOSTICS

The depletion of condensable elements onto grains in gaseous nebulae c an provide evidence that dust is well mixed with the ionized gas. Al a nd Ca are two of the most depleted elements in the general interstella r medium, and it is therefore important to measure their abundances wi thin the ionized region of nebulae. We compute a large grid of photoio nization models and identify sets of line ratios which are relatively insensitive to stellar and nebular parameters, and are thus excellent diagnostics for determining relative abundances. Based on the absence of the [Ca II] lambda lambda 7291, 7324 doublet and the detection of A l II] lambda lambda 2660, 2669 in the ultraviolet, we determine the ex tent of aluminum and calcium depletion onto grains in NGC 7027 and the Orion Nebula. Our results show a similar to 0.3 dex depletion for Al, but a depletion of more than two and a half orders of magnitude for C a. A similar calculation based on Mg II lambda 2798 yields roughly a 0 .8 dex depletion for Mg. This reaffirms the discrepancy between deplet ions determined from high and low ionization Mg lines. We also find ev idence for a ''depletion gradient'' in Ca in NGC 7027, since the calci um depletion we infer for the outer, more neutral regions using [Ca II ] is somewhat higher than that inferred for the inner high-ionization region, using [Ca V]. This gradient can test current models of the sur vival of grains within hot ionized gas.