Properties of Proto–Planetary Nebulae

This review describes some general properites of proto-planetary nebulae with particular emphasis on the recent work of morpholgical studies. The weight of observational evidence shows that proto-planetary nebulae (PPNe) are most certainly axisymmetric like planetary nebulae. Recent work suggests two subclasses of PPNe optical morphology, DUst-Prominent Longitudinally-EXtended (DUPLEX) and Star-Obvious Low-level Elongated (SOLE). Radiative transfer models of an example DUPLEX PPN and SOLE PPN, presented here, support the interpretation that DUPLEX and SOLE are two physically distinct types of PPNe. The DUPLEX PPNe and SOLE PPNe may well be the precursors to bipolar and elliptical PNe, respectively. 1. What is a Proto-Planetary Nebula? The proto-planetary nebula (PPN;a.k.a. post-AGB or pre-PN) stage of evolution immediately precedes the planetary nebula (PN) stage. The lifetime for this phase is < ∼ 1000 years and marks the time from when the star was forced off the asymptotic giant branch (AGB) by intensive mass loss to when the central star becomes hot enough (Teff ∼ 3× 10 4 K) to photoionize the neutral circumstellar shell (Kwok 1993). We refer the reader to Kwok (1993) for a comprehensive review of PPN. For short recent reviews, see Hrivnak (1997) and van Winckel (1999). In this short review, I summarize the basic properties of PPN but focus primarily on the morphological studies because there have been numerous morphological studies in the past few years and because this particular conference is focused on morphology. 2. General Characteristics of PPN PPN are like PN in that the central star illuminates a detached circumstellar shell, however, we observe PPN using quite different techniques. Because PPN do not have ionized gas, we can not use the optically bright emission lines or radio free-free continuum commonly used for studies of PN. Instead tracers of dust and neutral gas are employed. In fact, PPN are identified as stars of spectral type B-K, luminosity class I with infrared excesses. These infrared excesses arise from the circumstellar dust which was originally created in the AGB wind. They emit broad (∼20 km s), parabolic or double–horned lines of CO rotational lines and