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M. Meixner (University of Ilinois, Urbana-Champaign), T. Ueta (University of Illinois, Urbana-Champaign), M. Bobrowsky (Challenger Center for Science Education), A.K. Speck (University of Illinois, Urbana-Champaign)
We present radiative transfer models of the circumstellar dust in two proto-planetary nebulae (PPNe), which are objects evolving from the asymptotic giant branch to the planetary nebula phase. One of the PPNe, HD 161796, has been classified by us previously as a star-obvious low-level extended (SOLE) PPN (see Ueta, Meixner & Bobrowsky 2000). The second, IRAS 17150-3224, has been classified as a dust prominent longitudinally extended (DUPLEX) PPN. The radiative transfer code models the axisymmetric geometry observed in these two PPN. The constrained model parameters suggest that both objects experienced an axially symmetric superwind shortly before they departed from the asymptotic giant branch. However, the mass loss rates experienced by IRAS 17150-3224 were higher and more equatorially enhanced than those experienced by HD 161796. This difference in mass loss history explains the difference in morphological appearence of the two PPN. HD 161796, while having an almost edge-on torus, is almost transparent to the star light because of its lower optical depth in the dust. On the other hand, IRAS 17150-3224 has an optically thick equatorial torus which we view almost edge-on leaving the central star is completely obscured and allowing the starlight to escape in the beautiful bipolar reflection nebula.