Astrometric analysis of the homunculus of η carinae with the Hubble space telescope

Images of η Carinae, obtained with the HST Wide Field/Planetary Camera in 1990 October (WF1), 1991 April (PC1), and 1992 December (WF2) have been used to perform a detailed study of the proper motion of the homunculus of η Carinae. This analysis yields the plane-of-the-sky astrometric velocities which range from tens of kilometers per second to over 1000 km/sec with estimated uncertainties on the order of 40 km/sec. Our primary conclusion from these astrometric measurements is that the motion of the homunculus of η Carinae is largely radial, increasing linearly with distance from the central star. We measure an average 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 measurements, respectively. These deviations are the computed standard deviation from linear expansion. The deviation between the two comparisons is 12 mas. Thus we believe the deviations seen in the comparison pairs to be somewhat correlated, implying that some of the non-linearities in the expansion are real. Our direct measurements imply a single eruptive event centered in 1841.2±0.8 years (standard deviation of mean) or ±4 years when one includes some corrections in the error estimate for the correlated motions and relative plate scale errors. This agrees well with the historical "Great Eruption" which peaked in 1843. The motion of the individual fragments indicates "times of ejection" for the fragments occurred over an interval of less than 20 years. We include astrometric measurement of the North "Jet" containing the NN and NS knots and find the knots generally follow the linear radial 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), rather than a part of a continuing jet. Finally, we demonstrate that astrometric measurements of extended objects with the Hubble Space Telescope (pre- and post-repair) are feasible at the 5-10 mas level (≈ 20-40 km/sec at 2 kiloparsecs).