Mid‐Infrared (8–21 micron) Imaging of Proto–Planetary Nebulae

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Abstract
We present mid-infrared (8-21 μm) images of thermal dust emission from two proto-planetary nebulae (PPNs), IRAS 07134+1005 and IRAS 22272+5435, which show a strong 21 μm emission feature. Both of the sources are well resolved and show evidence for axial symmetry. From our images, we calculate temperature and optical depth maps and estimate the abundance of the 11 μm and 21 μm feature carriers. In both sources, the dust temperatures range from ~160-200 K. The optical depths in IRAS 07134 are about a factor of 3 lower than those in IRAS 22272, but the emission is optically thin in both sources. Our analyses of the feature-to-continuum ratios suggests that 0.5%-5% of the carbon in these objects may be in the form of large PAH molecules. We construct optically thin, axially symmetric cylindrical shell models to simulate the observed mid-IR morphologies and spectra, and calculate nebular masses of 0.26 M☉ for IRAS 07134 and 0.42 M☉ for IRAS 22272. Although the mid-IR emission primarily comes from warm (T ≈ 190 K) dust, our models require a significant cooler dust (T ≈ 80 K) component to fit the observed mid- and far-IR spectral energy distributions.