A SURVEY OF PLANETARY-NEBULAE AT NA-D - EVIDENCE FOR NEUTRAL ENVELOPES

We present high-dispersion spectra of 21 planetary nebulae in the regi on of the Na I D lines at lambda lambda 5889, 5895. These reasonance l ines, long used to study neutral interstellar clouds, act as tracers o f cool, neutral material associated with the ionized nebulae. Over hal f the observed objects show nebular components in the Na D lines, eith er in absorption or emission, or both; some of these were not previous ly suspected to contain neutral material. For objects in which well-re solved absorption components are seen in the Na I lines, we estimate c olumn densities using the doublet-ratio method, finding N(Na I) approx imate to 10(11-13) cm(-2). For five objects previously detected at 21 cm, we estimate the ratios N(Na I)IN(H I). The values for the planetar y nebulae are somewhat higher than for typical interstellar clouds, in dicating that, integrated through the absorbing column, a larger fract ion of the sodium is neutral. The layers where the Na I lines are form ed generally appear to be expanding at similar velocities as the ioniz ed gas, a result previously found for H I. A couple of objects show bl ue wings in the nebular Na absorptions, possibly indicating accelerati on, nonspherical ejection, or the presence of clumps of low-ionization , fast-moving material. Our results imply that the outer portions of m any planetary nebulae are neutral. Therefore, the nebulae are radiatio n-bounded rather than mass-bounded, and the total masses are larger th an the ionized masses. Resonance lines such as the Na I lines offer a means of detecting circumnebular material whether or not molecules are present, and of investigating properties of the neutral envelopes suc h as ionization balance, column densities, and kinematics.