HST Imaging of Proto-Planetary Nebulae and Very Young Planetary Nebulae-Towards a New Understanding of Their Formation

Abstract

With the advent of the Wide Field Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope (HST) in Dec 1993, a growing number of young planetary and proto-planetary nebulae (PNe and PPNe) have been imaged with unprecedented high angular resolution and dynamic range. These objects represent the last phases of the evolution of intermediate-mass (~1–8 M ⊙) stars. Our WFPC2 Hα SNAPshot imaging survey of young planetary nebulae (selected only on the basis of their low excitation) show that these objects are highly aspherical, with complex multipolar morphologies. The central star is often systematically offset from the geometrical symmetry centers of various nebular components. The bright aspherical nebulae are often found to be surrounded by faint, roughly round halos — signatures of the progenitor AGB envelopes produced by isotropic mass-loss. A number of these halos include numerous concentric arcs, evidence for quasi-periodic modulation of the mass-loss on time-scales of a few hundred years. Our detailed imaging studies of PPNe (objects in transition between the AGB and PN evolutionary phases) shows the emergence of complex morphological structures during the PPN phase. The complexity, organization and symmetry of the morphological structures we find is forcing radical changes in, and inspiring fresh theoretical efforts to advance, our understanding of the mass-loss processes during late stellar evolution. In this paper, we review the HST data, and show some of the highlights of our imaging studies. Although the origins of many of the morphological features remains puzzling, we find that the data support a model for PN formation in which the primary agent for shaping PNe are high-speed collimated outflows or jets which operate during the late AGB and/or early post-AGB evolutionary phase. The multipolar shapes indicate that these outflows are bipolar and undergo episodic changes in their orientation and/or multiple collimated outflows occur with different orientations. Our discovery of a very highly-collimated, knotty bipolar jet in a planetary nebula, and its amazing morphological similarity to a low-mass YSO provides strong empirical evidence for a common physical mechanism for generating collimated outflows in protostars and evolved stars.