The Angular Expansion and Distance of the Planetary Nebula BD +30°3639

Abstract

The WFPC2 camera aboard the Hubble Space Telescope was used to obtain images of the planetary nebula BD +30°3639 at two epochs separated by 5.663 yr. The expansion of the nebula in the Hα and [N II] bands has been measured using several methods. Detailed expansion maps for both emission lines were constructed from nearly 200 almost independent features. There is good agreement between the (independent) Ha and [N II] proper motions. There are clear deviations from uniform radial expansion, with higher expansion rates in regions where the shell is faintest, such as the southwest quadrant. The Space Telescope Imaging Spectro-graph was used to obtain echelle spectra in the C II] λ2326 multiplet and the [O II] λ2470 doublet, providing well-resolved expansion velocities at two position angles. From the C II] lines we find that the central velocity split is ±36.3 km s-1 at a position angle (P.A.) of 99° and ±33.5 km s-1 at P.A. 25°. The fainter [O II] doublet does not appear to differ from the [O II] multiplet. To determine the distance of BD +30° 3639 by comparison of the angular expansion and the spectroscopically determined radial expansion, we must address the problem of the three-dimensional shape of the nebula. We measured the angular expansion along the position of the 99° echelle slit, finding displacements of 4.25 mas yr-1 at the shell edge (2″. 47 from the center). If the nebula were spherical, this would imply a distance of 1.80 kpc. But there is evidence that the nebula is elongated along the line of sight, which suggests that the actual distance is less. Radio continuum images from 5 and 15 GHz VLA observations provide information on the extent of the radial elongation. We fit the radio brightness, variation and the echelle data by approximating the nebula as an ellipsoid, also making use of the ground-based echelle spectra reported by Bryce & Mellema. Our model has an axial ratio of 1.56, is inclined to the line of sight by 9°.7, and exhibits an expansion in the plane of the sky which is 2/3 that in the radial direction, leading to a distance of 1.2 kpc. Not all the kinematic data fit this simple model, so the distance must still be regarded as uncertain. Based on the recent model atmosphere of Crowther et al., a distance of 1.2 kpc implies a stellar luminosity of 4250 L⊙. The kinematic age of the nebula, θ/θ, varies somewhat from region to region. A good average value is 800 yr, while the expansion along the position of the 99° echelle slit gives about 600 yr.