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M. Meixner, T. Ueta (University of Illinois), M. Bobrowsky (Orbital Sciences Corporation)
We present the idea that there are two types of proto-planetary nebulae (PPNe) which are objects in transition from the asymptotic giant branch to planetary nebulae phase of intermediate mass star evolution. The two types, which we will call optically-thin and optically-thick, differ in their observed mid-infrared (mid-IR; 8-24 microns) morphology, optical (V and I band) morphology and spectral energy distributions. In the mid-IR, the optically-thin PPNe show two limb-brightened peaks of a torus, while the optically-thick PPNe show unresolved compact cores surrounded by lower surface brightness elliptical nebulae. Meixner et al. 1998 distinguished these two types of mid-IR morphology calling the optically-thin PPNe, toroidals, and the optically-thick PPNe, core/ellipticals. Our new HST optical images also show distinct differences in two types of PPNe. The optically-thin PPNe have very prominent central stars surrounded by very faint elliptical and multipolar reflection nebulae. The optically-thick PPNe show spectacular bipolar reflection nebulae with little or no direct viewing of the central star. The spectral energy distributions of the optically-thin PPNe show two distinct peaks: one in the optical/near-infrared caused by the central star and one in the infrared caused by the circumstellar dust shell. The spectral energy distributions of the optically-thick PPNe show a prominent peak in the infrared caused by the circumstellar dust and a faint excess in the optical/near-infrared due to the reflection nebulosity. As examples of these two types of PPNe, we present new HST data of IRAS 17436+5003 (a.k.a. HD161796), an optically-thin PPN, and IRAS 17150-3224, an optically-thick PPN.