A Survey of Internal Motions in the Planetary Nebulae.

Slit spectrograms (dispersion 10 A/mm) of twenty-six bright planetary nebulae have been obtained with the coude' spectrograph of the 100-inch telescope. For thirteen of the nebulae, slitiess spectrograms have been secured with the same instrument. he main observational results are the separations of the components of the double nebular lines and the dimensions of corresponding slitless images. Analysis of these data leads to the following principal conclusions: 1. There is no correlation between the relative intensities (V/R) of the red and violet components of the nebular lines and the general level of nebular excitation. 2. The number of nebulae with V/R> 1 is about equal to the number with V/R < 1; hence the nebulae are probably transparent in the observed wave4ength range. 3. The most reliable measure of expansion velocity is shown to be the mean of the values given by lines of H, 0 ni, and Ne in. There may be a very weak positive correlation between this quantity and the level of excitation. 4. Where differences in component separation exist in the spectrum of a planetary nebula (and they are quite common), tife particles show smaller separations than the low-excitation particles. Extreme examples are Ne v, for which the separations are often close to zero, and 0 n and N n, which sometimes show large separations. Hydrogen presents an outstanding exception to the general run of separation with excitation, agreeing closely with 0 in and Ne in. Available evidence is against any marked dependence of component separation upon transition probability. 5. There is generally a positive correlation between component separation and monochromatic image size. Usually, though not always, differences in image size are much less than differences in separation. 6. Components of the nebular lines are often quite sharp, their apparent width being frequently determined by the resolving power of the emulsion; but components of H lines are noticeably diffuse in all nebulae. By-products of the investigation include improved wave lengths of twenty nebular lines and radial velocities of twelve nebulae. Two models have been investigated in attempting to explain the observations. The basic assumption of the first model is that the velocity given by the lines of any ion is a measure of the velocity of the abundant H and He in the region where the lines in question are produced. In this model the material is supposed to have originally a velocity equal to that measured for Ne v, which is, in some nebulae, probably less than 5 km/sec. The increments in velocity between Ne v and He ii and between He ii and H are assumed to be due to the increases in momentum flux carried by the gas upon absorption of nuclear radiation below x 228 A and between x 912 and x 228 A, respectively. Reasonable values of the radius and temperature of the central star lead, on this model, to the correct orders of magnitude both for the velocity changes and for the total nebular brightness. The second model assumes that measured velocity differences are due to selective forces acting upon some of the rare ions and maintaining them in relative motion with respect to the abundant H and He. It appears possible in this fashion to construct a model which can account for the Ne v anomaly on the basis of an inward flux of Lyman continuum from the bright hydrogen shell. This model leads, however, to nebulae about 5 mag. too faint, and for this reason the first one seems preferable.