ASTROMETRIC ANALYSIS OF THE HOMUNCULUS OF ETA-CARINAE WITH THE HUBBLE-SPACE-TELESCOPE

Images of eta Carinae, obtained with the HST Wide Field/Planetary Came ra in 1990 October (WF1), 1991 April (PC1), and 1992 December (WF2) ha ve been used to perform a detailed study of the proper motion of the h omunculus of eta Carinae. This analysis yields the plane-of-the-sky as trometric velocities which range from tens of kilometers per second to over 1000 km/sec with estimated uncertainties on the order of 40 km/s ec. Our primary conclusion from these astrometric measurements is that the motion of the homunculus of eta Carinae is largely radial, increa sing linearly with distance from the central star. We measure an avera ge radial expansion rate of 0.66% per year. The deviations from a pure linear expansion are 12 mas and 17 mas for the PC1:WF2 and WF1:WF2 me asurements, respectively. These deviations are the computed standard d eviation from linear expansion. The deviation between the two comparis ons is 12 mas. Thus we believe the deviations seen in the comparison p airs to be somewhat correlated, implying that some of the non-linearit ies in the expansion are real. Our direct measurements imply a single eruptive event centered in 1841.2+/-0.8 years (standard deviation of m ean) or +/-4 years when one includes some corrections in the error est imate for the correlated motions and relative plate scale errors. This agrees well with the historical ''Great Eruption'' which peaked in 18 43, The motion of the individual fragments indicates ''times of ejecti on'' for the fragments occurred over an interval of less than 20 years . We include astrometric measurement of the North ''Jet'' containing t he NN and NS knots and find the knots generally follow the linear radi al expansion rate of the homunculus. The NN and NS knots are ''bullets '' emitted at the time of the eruption (or up to 10 years later), rath er than a part of a continuing jet. Finally, we demonstrate that astro metric measurements of extended objects with the Hubble Space Telescop e (pre- and post-repair) are feasible at the 5-10 mas level (approxima te to 20-40 km/sec at 2 kiloparsecs). (C) 1996 American Astronomical S ociety.