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J. Li, J. P. Harrington (University of Maryland), K. J. Borkowski (North Carolina State University)
The HST WFPC2 was used to obtain images of BD+30\circ3639 through the H\alpha and [N~II] narrow-band filters at two epochs 5.663 years apart. The expansion of the nebula was measured using a variety of methods. Detailed expansion maps for both emission lines have been constructed from nearly 200 almost independent features. The agreement of the (independent) H\alpha and [N~II] proper motions inspires confidence in the accuracy of the measurements. There are clear deviations from uniform radial expansion, with higher expansion rates in the south-west quadrant.
Echelle spectra in C~II] \lambda2326 were obtained with the HST STIS, providing well-resolved expansion velocities at two position angles. We find that a central velocity of ±36.3 km s-1 at p.a. 99\circ, and ±33.5 at p.a. 25\circ.
We measured the angular expansion along the position of the 99\circ echelle slit, finding displacements of 4.4 mas yr-1 at the shell edge (2.5" from the center). For a spherical nebula, this would imply a distance of 1.75 kpc, but there is evidence that the nebula is elongated along the line of sight, implying that the actual distance is less. VLA radio continuum images provide information on the extent of the elongation. We fit the radio brightness variation and the echelle data by approximating the nebula as an ellipsoid. Our model has an axial ratio of 1.5, inclined to the line of sight by 11\circ, with an expansion in the plane of the sky 70% that in the radial direction, leading to a distance of 1.2 kpc. The kinematic age, \theta/\dot{\theta}, averages 800 yr, but varies and is 600 yr along the echelle slit.
Support for this work was provided by NASA through grant GO-08116.01-97A from STScI.