High-spatial-resolution observations of dust and gas in NGC 7027 and the M8 hourglass

The spatial relationships among the distribution of the ionized gas, continuum dust emission, and the unidentified 3-12 μm infrared dust feature emission yield important clues to the identification of candidate grain/molecule materials responsible for feature emission, and provide observational constraints useful in the evaluation of proposed emission mechanisms. Here the results are presented of a systematic study of the 3.28-μm and 3.4-μm dust emission features, two of the unidentified emission bands' frequently detected in the 2-4 μm band of a variety of astronomical sources, in two energetically and morphologically dissimilar objects: the planetary nebula NGC 7027 and the M8 Hourglass HII region. Comparison and analysis of the moderate spectral resolution (Δλ/λ ∼ 1%), high spatial resolution (∼ 0.4''/pixel) infrared images of NGC 7027 in the 3.28-μm dust-emission feature and the Brα (4.052 μm) hydrogen recombination line clearly show that the 3.28-μm emission is spatially more extended than that of the ionized gas. Most of the 3.28-μm feature emission arises from a thin shell, ∼ 1'' thickness, at least partially outside of the HII region. Based on a simple model for the source morphology, we conclude that the 3.28-μ emitters are quite robust, because they are able to survive the extremely hard ultraviolet radiation field at the interior edge of the HII region