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K. Davidson (U. Minnesota), T.R. Gull, K. Ishibashi (NASA/GSFC), D.J. Hillier (U. Pittsburgh)
Doppler velocities can be used to trace a cross-section of the bipolar ``Homunculus'' nebula of material ejected from \eta Car in the giant eruption seen 160 years ago. However, normal ground-based observations have not had sufficient spatial resolution for this task. Data obtained with HST/STIS in March 2000 now provide the first satisfactory results.
The configuration's inclination or tilt can be measured reliably, for the first time, by using velocities in the equatorial debris-disk. We find that the angle between bipolar axis and line of sight is close to 41 degrees.
Even with excellent data, the bipolar lobe shape is intrinsically difficult to measure. A shape resembling a hot-air balloon fits the data best. Earlier descriptions as ``osculating spheres'' or ``flask-like shapes'' or ``polar caps'' each contain limited elements of truth. The outer or polar parts of each lobe appear to contain more mass than the lobe sides.
Our data show interesting structure near the equatorial mid-plane, probably more important than the lobe shapes. In addition to equatorial debris from the great eruption of the 1840's, velocities corresponding to a later ejection time, around 1900, are also present. There are hints of pre-1840 equatorial ejecta too, but these are uncertain.
This work is supported by NASA through grant GO-8327 from the STScI.