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We present observations of selected regions of the ultraviolet spectra of four planetary nebulae, made in the medium-resolution mode of the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope. Up to 7 spectral segments between 1200 and 1700 $\AA$ were observed for the nebulae NGC 40, IC 418, IC 3568, and BD+30$\deg$3639. These nebulae were chosen because they have relatively bright central stars, and were previously found to show circumstellar Na I absorption features (Dinerstein $\&$ Sneden 1988, ApJ 335, L23; Dinerstein, Sneden, $\&$ Uglum 1990, BAAS 22, 1271; and 1994, submitted). The GHRS spectra show a number of resonance absorption lines from foreground interstellar material, including low-ionization species such as O I, C II, C I and Si II. In some cases we also see circumnebular absorption components, located at the Doppler shift corresponding to the forward side of the expanding nebular shell. Some of these features are presumed to be formed in predominantly neutral material adjacent to the ionized gas. Of the observed nebulae, BD+30$\deg$3639 has the strongest circumstellar absorption features. The ratios of the strengths of lines arising from different fine-structure levels of O I and C I are indicative of a higher temperature in the circumnebular gas than in the ambient interstellar medium. Analysis of the spectra can provide information about the conditions in and amount of material in the neutral envelopes around planetary nebulae.
This research was supported by Space Telescope Science Institute grant GO 3880.01-91A and NSF grant AST 91-15101.
