One of the major uncertainties in our understanding of grains is their composition, and how the composition and abundance of dust vary with environment. In most cases, the depletions show a dependence on the average gas density along the line of sight. It is also possible to observe emission lines from ions of refractory elements in ionized nebulae, from which one can infer the properties of grains in the ionized gas and study how grains may be modified in the vicinity of hot stars. Filled circles represent our FOS measurements. Open squares represent abundances in the Orion Nebula from IUE spectra.
Symbols are the same as above.
There are few measurements of refractory element depletions in HII regions for comparison. These depletions are consistent with our measured Si depletions, if the trend of Fe depletion as a function of Si depletion shown by Sofia et al. Thuan et al. The fact that we see an average Si depletion of only -0.3 dex in the observed HII regions suggests that the grains in the ionized gas have been modified. Therefore, it may be that shocks are modifying the grains within the HII region.