A Survey of Planetary Nebulae at NA D: Evidence for Neutral Envelopes

We present high-dispersion spectra of 21 planetary nebulae in the region of the Na I D lines at λλ5889, 5895. These resonance lines, long used to study neutral interstellar clouds, act as tracers of cool, neutral material associated with the ionized nebulae. Over half the observed objects show nebular components in the Na D lines, either in absorption or emission, or both; some of these were not previously suspected to contain neutral material. For objects in which well-resolved absorption components are seen in the Na I lines, we estimate column densities using the doublet-ratio method, finding N(Na I) ≍ 1011-13 cm-2. For five objects previously detected at 21 cm, we estimate the ratios N(Na I)/N(H I). The values for the planetary nebulae are somewhat higher than for typical interstellar clouds, indicating that, integrated through the absorbing column, a larger fraction of the sodium is neutral. The layers where the Na I lines are formed generally appear to be expanding at similar velocities as the ionized gas, a result previously found for H I. A couple of objects show blue wings in the nebular Na absorptions, possibly indicating acceleration, nonspherical ejection, or the presence of clumps of low-ionization, fast-moving material. Our results imply that the outer portions of many planetary nebulae are neutral. Therefore, the nebulae are radiation-bounded rather than mass-bounded, and the total masses are larger than 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 such as ionization balance, column densities, and kinematics.