The discovery of circumstellar arcs around two bipolar protoeplanetary nebulae

Circumstellar arcs have been discovered using the Hubble Space Telescope in two proto-planetary nebulae (PPNs), IRAS 16594-4656, and IRAS 20028+3910. This raises to five the number of PPNs in which such arcs have been detected and suggests that they are not rare and may even be common features. All five of these PPNs display a bipolar morphology, but this correlation may be the result of a selection effect in which deeper images are taken of bipolar PPNs in which the central star is obscured. Two pairs of oppositely directed "searchlight beams" are seen in IRAS 20028+3910, similar to those seen in AFGL 2688 and IRAS 17150-3224. The presence of circular arcs and rings in PPNs and planetary nebulae with a variety of viewing orientations implies that they are illuminated portions of spherical shells viewed in scattered light. This suggests that the mass-loss process is spherically symmetric over most of the lifetime of the progenitor asymptotic giant branch (AGB) star, and the transition to a bipolar morphology occurs over a short period at the end of the AGB evolution.