Planetary Nebulae Expansion Distances. II. NGC 6572, NGC 3242, and NGC 2392

As part of a continuing program to estimate the distances to planetary nebulae (PNe), we have applied an expansion parallax algorithm to four objects: NGC 2392, NGC 3242, NGC 6210, and NGC 6572. The algorithm requires two Epochs of data obtained at the VLA at λ6 cm. With 6 years between Epochs, we have measured the angular expansion rate of three of the above PNe, NGC 6572, NGC 6210, and NGC 3242 with errors of 20, 25, and 40%, respectively. We did not detect the expansion of NGC 2392, but were able to ascertain an upper limit to the expansion rate. By combining the detected angular expansion rate with the Doppler expansion velocity and assuming uniform expansion, we have directly computed the distances to NGC 6572, NGC 6210, and NGC 3242 with accuracies that represent significant improvements over the current estimates in the literature. For most of the nebulae thus surveyed, our expansion parallax distances are all at least as large as recent Shklovsky distances determined by statistical methods.