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R. Sahai (Jet Propulsion laboratory/ Caltech), S. Brillant, W. Brandner (ESO), M. Livio (STScI), L.-A. Nyman (Onsala Space Observatory & ESO), S. Tingay (Australia Telescope National Facility)
The drastic changes observed in circumstellar structure and kinematics [from spherical, slowly expanding AGB envelopes to bipolar planetary nebulae (PNe) with high polar velocities] have been a long-standing puzzle for theories of late stellar evolution. Recently, based on results of an unbiased survey of young PN morphologies using HST, Sahai & Trauger (1998, AJ, 116, 1357) have hypothesised that high-speed jet-like outflows are the primary agent for producing these changes. One of the objects in this survey, Hen\,2-90, was found to possess a highly collimated bipolar jet and a central bipolar nebula bisected by a flaring disk-like structure (Sahai & Nyman 2000, ApJ, 538, L145). The jet showed at least six pairs of emission knots located symmetrically on either side of the nebular center, with a tangential speed of ~150\,km\,s-1 estimated from the jet opening angle.
We report here the succesful detection of proper motions of the knots by re-imaging this object with HST two years after its discovery. The average proper motion of the knots is 0.06 arcsec yr-1; using ground-based spectroscopy, we find that the red- and blue-shifted sides of the bipolar jet show a constant radial outflow velocity of ±26\,km\,s-1. From these results, for an estimated distance of 2.5 kpc to Hen\,2-90, we derive an intrinsic jet speed of 360\,km\,s-1. Deep ground-based [NII] and H\alpha images show that the jet extent, which is surface-brightness limited, is about 40 arcsec on either side of the center. Using our measured proper motion of the knots, we find that the minimum jet lifetime is 1400 yrs, and that the knots are being ejected at the rate of one pair every 40 years (independent of the tilt angle and distance to Hen\,2-90). We examine the implications of these results for the nature and origin of the enigmatic jet in Hen\,2-90, which so far appears to be a unique object.
The author(s) of this abstract have provided an email address for comments about the abstract: sahai@bb8.jpl.nasa.gov