The Spectra of Thirty-Three Gaseous Nebulae in the Yellow-Green Region Obtained with an Electronic Camera.
We describe observations of thirty-two planetaries and the Orion Nebula, carried out with electronic cameras at the coude focus of the Lick 120-inch reflector. Advantages of this combination are a large scale and a relatively high speed in the spectral range 4600-5900 A. Throughout much of this range, photographic emulsions are discouragingly slow. A number of astrophysically important lines of He I, He ii, C iv, [N iJ, [N ii], 0 iii, [0 iii], Si ii, [Cl iii], [Cl iv], [Ar iii], [Ca v], [Fe iii], [Fe vi], and [Fe vii] fall in this region. The observational results are presented in a series of tables with appended notes. Certain nebulae, notably NGC 6543, NGC 2392, and IC 2003 show striking small-scale changes in excitation, whereas NGC 6720 and possibly NGC 7662 and 7009 tend to show more orderly stratification effects. The [N i] and [N ii] lines are found to be especially sensitive indicators of low-excitation regions. Knots of these radiations may be compared to prominences in the solar corona, although it is not established that the [N I] regions have substantially higher densities or lower temperatures. It is concluded that future studies of planetaries must emphasize detailed structural features and the construction of three-dimensional models in addition to their "integrated" properties.