Mid-infrared (8-21 micron) imaging of proto-planetary nebulae

We present mid-infrared (8-21 mu m) images of thermal dust emission from two proto-planetary nebulae (PPNs), IRAS 07134+1005 and IRAS 22272+5435, which show a strong 21 mu m 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 mu m and 21 mu m feature carriers. In both sources, the dust temperatures range from similar to 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 approximate to 190 K) dust, our models require a significant cooler dust (T approximate to 80 K) component to fit the observed mid-and far-IR spectral energy distributions.