Two Subclasses of Proto-Planetary Nebulae: Model Calculations

We use detailed radiative transfer models to investigate the differences between the Star-Obvious Low-level-Elongated proto-planetary nebulae (SOLE PPNs) and DUst-Prominent Longitudinally-EXtended proto-planetary nebulae (DUPLEX PPNs) which are two subclasses of PPNs suggested by Ueta, Meixner & Bobrowsky (2000). We select one SOLE PPN, HD 161796, and one DUPLEX PPN, IRAS 17150−3224, both of which are well studied and representative of their PPN classes. Using an axisymmetric dust shell radiative transfer code, we model these two sources in detail and constrain their mass-loss histories, inclination angles and dust composition. The physical parameters derived for HD 161796 and IRAS 17150−3224 demonstrate that they are physically quite different and that their observed differences cannot be attributed to inclination angle effects. Both HD 161796 and IRAS 17150−3224 are viewed nearly edge-on. However, the more intensive axisymmetric superwind mass loss experienced by IRAS 17150−3224 (8.5×10 M⊙ yr −1 and an Ṁequator/Ṁpole = 160) has created a high optical depth dust torus (AV=37) which obscures its central star. In contrast, HD 161796, which underwent a lower rate superwind (Ṁ= 1.2× 10 M⊙ yr and an Ṁequator/Ṁpole = 9), has an optically thinner dust shell which allows the penetration of direct star light. Based on our analysis of the dust composition, which is constrained by dust optical constants derived from laboratory measurements, both objects contain oxygen rich dust, mainly amorphous silicates, but with some significant differences. IRAS 17150−3224 contains only amorphous