Interferometric observations of OH and H2O masers in protoplanetary nebulae imaged with HST - A unique diagnostic of their spatial-kinematic structure

Abstract

One of the most exciting challenge facing theories of post-main sequence evolution today is to understand how Asymptotic Giant Branch (AGB) stars transform themselves into aspherical planetary nebulae (PNe). Recently, high-resolution imaging surveys of young planetary nebulae and protoplanetary nebulae (PPNe: objects in transition between the AGB and PN phases) have revealed that the majority of these objects are characterised by multipolar bubbles distributed roughly point-symmetrically around the central star. These data strongly suggest that the current model for the shaping of PNe is no longer adequate. High angular-resolution kinematic information is sorely needed to complement the imaging data in order to test new hypotheses, such as our proposal that episodic high-speed jet-like outflows, operating during the protoplanetary or very late-AGB phase, are the primary agent.We have therefore begun a program of using interferometric mapping of OH (and H2O, when feasible) maser emission in order to trace the kinematics of the structures discovered in protoplanetary nebulae with HST. These masers provide a unique and crucial probe of the kinematics of the circumstellar material in PPNe, because of the lack of other emission-line diagnostics. Although our work is still in its infancy (only two objects have been studied in detail), we find that the OH masers indicate the presence of multiple low-latitude outflows and an increase of outflow velocity with latitude. This paper summarises our progress so far, the state of current studies, and future prospects.