Preplanetary Nebulae: An HST Imaging Survey and a New Morphological Classification System

TL;DR

This study used HST imaging to classify preplanetary nebulae morphologies, revealing diverse aspherical structures and halos, supporting the idea that aspherical shaping begins before the planetary nebula phase.

Contribution

The paper introduces a comprehensive morphological classification system for preplanetary nebulae based on HST images, highlighting early aspherical shaping processes.

Findings

Discovery of well-resolved geometrical structures in PPNs

Identification of halos indicating undisturbed circumstellar envelopes

Evidence that aspherical structures begin forming during the PPN phase

Abstract

Using the Hubble Space Telescope (HST), we have carried out a survey of candidate preplanetary nebulae (PPNs). We report here our discoveries of objects having well-resolved geometrical structures, and use the large sample of PPNs now imaged with HST (including previously studied objects in this class) to devise a comprehensive morphological classification system for this category of objects. The wide variety of aspherical morphologies which we have found for PPNs are qualitatively similar to those found for young planetary nebulae in previous surveys. We also find prominent halos surrounding the central aspherical shapes in many of our objects -- these are direct signatures of the undisturbed circumstellar envelopes of the progenitor AGB stars. Although the majority of these have surface-brightness distributions consistent with a constant mass-loss rate with a constant expansion velocity, there are also examples of objects with varying mass-loss rates. As in our surveys of young planetary nebulae (PNs), we find no round PPNs. The similarities in morphologies between our survey objects and young PNs supports the view that the former are the progenitors of aspherical planetary nebulae, and that the onset of aspherical structure begins during the PPN phase (or earlier). Thus, the primary shaping of a PN clearly does not occur during the PN phase via the fast radiative wind of the hot central star, but significantly earlier in its evolution.