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R.S. French, H.L. Dinerstein, C. Sneden (U. Texas)
The presence of cool, neutral gas in interstellar space was initially proven by seeing absorption lines of low-ionization species such as Na~I, Ca~II, and Ti~II projected on the spectra of background stars. Dinerstein and Sneden (1988, ApJ, 327, L27) used this technique on the central star of the planetary nebula (PN) BD+30~3639, and discovered absorption line components at the radial velocity expected for the blue edge of the expanding shell. The circumnebular origin of these features was conclusively demonstrated by the presence of emission components at off-star positions (a spatially-resolved P Cygni profile!). In a follow-up survey including 20 additional PNe, Dinerstein, Sneden & Uglum (1995, ApJ, 447, 262) found evidence for nebular features in more than half the sample. We have now obtained higher-resolution (R = 60,000) spectra of several dozen PN central stars, using the ``2-d coudé" spectrometer on the McDonald Observatory 2.7m telescope. These cross-dispersed echelle spectra cover the spectral region 3700--9500 Å, including the Na~I D lines at 5890, 5896~Å\ and weaker lines of other low-ionization and molecular species. Here we present our new measurements of the Na~I lines and other circumstellar features that must arise in the neutral and molecular layers of the nebular envelope (which can also be regarded as a ``photodissociation region"). The equivalent widths of unsaturated absorption lines, and of doublet lines such as those of Na~I, K~I, and Ca~II, yield information on the column densities of various species in the neutral envelope, which, in turn, provide constraints on the envelope properties and masses. The advantage of our higher spectral resolution is evident in our new spectrum of BD+30~3639: what appeared as a barely noticeable enhanced blue wing of the Na I lines in our earlier data is now seen as a distinct second absorption system blueshifted by an additional ~q18~km/sec. In addition, we have found new examples of PNe with strong circumstellar features (e.g. IC 5117).