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We identify several emission line diagnostics which readily determine whether grains are present in ionized gas. These are based on the fact that Ca and Al have among the highest gas phase depletions in the interstellar medium. We show that lines of these elements would be among the strongest in the spectrum of a photoionized gas were their gas phase abundances equal to the solar value.
Based on the absence of the [Ca~II] $\lambda\lambda$7291,7324 doublet and the detection of Al~II] $\lambda\lambda$2660,2669 in the ultraviolet, we determine the extent of aluminum and calcium depletion onto grains in NGC~7027 and the Orion Nebula. Our results show a $\sim$0.3 dex depletion for Al, but a depletion of more than two and a half orders of magnitude for Ca. A similar calculation based on Mg~II $\lambda$2798 yields roughly a 0.8 dex depletion for Mg. This reaffirms the discrepancy between depletions determined from high and low ionization Mg lines. We also find evidence for a ``depletion gradient'' in Ca in NGC~7027, since the calcium 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 survival of grains within hot ionized gas.
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