Old planetary nebulae and the relation between size and expansion velocity.

A plot of data on expansion velocity of [0 III] ions as a function of outer radius for relatively small, high- surface-brightness planetary nebulae is consistent with a linear relation between the two parameters. For medium- and high-excitation planetaries, this relation also holds for hydrogen ions and deviates significantly from the relation of Mathews (derived for idealized spherically symmetric nebulae). Extrapolation of this empirical relation to larger radii would require very large expansion velocities for the old planetary nebulae and would thereby raise questions regarding the energy needed to drive and maintain the expansion. Observations of emission-line profiles from a few large, faint, and presumably old planetary nebulae were made in order to test the validity of such an extrapolation. The new data show that at least some of the older planetary nebulae have unusually low intrinsic velocities of expansion. These results suggest that although adjustments to the assumed time-dependent dynamical pressure may be required in Mathews's models, these models can account, at least qualitatively, for the general expansion properties of the planetary nebulae. Subject headings: planetary nebulae - gas dynamics - radial velocities - stellar evolution