MIDINFRARED (8-21 MICRON) IMAGING OF PROTOPLANETARY NEBULAE

We present mid-infrared (8-21 mu m) images of thermal dust emission fr om two proto-planetary nebulae (PPNs), IRAS 07134+1005 and IRAS 222725435, which show a strong 21 mu m emission feature. Both of the source s are well resolved and show evidence for axial symmetry. From our ima ges, 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 d epths in IRAS 07134 are about a factor of 3 lower than those in IRAS 2 2272, but the emission is optically thin in both sources. Our analyses of the feature-to-continuum ratios suggests that 0.5%-5% of the carbo n in these objects may be in the form of large PAH molecules. We const ruct optically thin, axially symmetric cylindrical shell models to sim ulate the observed mid-IR morphologies and spectra, and calculate nebu lar masses of 0.26 M. for IRAS 07134 and 0.42 M. for IRAS 22272. Altho ugh the mid-IR emission primarily comes from warm (T approximate to 19 0 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.