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Spectroscopic observations of the central star of the planetary nebula Abell 78 have been obtained with the GHRS onboard the Hubble Space Telescope (HST). We observed the central star in the vicinity of the C~IV~$\lambda$1548.2,1550.8~\AA~doublet.
We find a series of narrow absorption features superimposed on the broad, P-Cygni stellar wind profile. These features are seen in both components of the doublet, and have velocities of $-23$, $-74$, $-133$ and $-200$~km/s. At higher velocities, individual components are no longer distinct, but merge into a continuous absorption extending to $\sim$375 km/s. This is among the highest velocities ever detected for gas in a planetary nebula.
Our HST [O~III]~$\lambda$5007~\AA~image of the central region of this object revealed "cometary" structures extending radially away from the star (Borkowski et al., ApJ 415, L47, 1993). We have interpreted these tails as evidence of mass-loading of the stellar wind as it flows past dense condensations of hydrogen-poor ejecta. The C~IV~$\lambda$1549~\AA~absorption features can then be readily explained as due to parcels of ejecta being accelerated to high velocities as they are swept up by the wind.
Some of these absorption features had been seen previously with poorer S/N in IUE spectra (Kaler et al., ApJ 324, 528, 1988). The $-23$ and $-74$~km/s absorption components are optically thick, but the optical depths of the $-133$ km/s components are $\sim$0.9 and $\sim$1.8, respectively. The implied C~IV column density at this velocity is $8 \times 10^{14}$~cm$^{-2}$. We estimate the total C$^{+3}$ column along our line of sight to be $\geq 5 \times 10^{15}$~cm$^{-2}$.
An irregular bubble can be seen surrounding the inner knot region in A78. As the ablated material is accelerated, it will mix with the stellar wind; the bubble most likely results when this mass-loaded flow is halted by the H-rich envelope. The abundance anomalies seen at the rim of the bubble attest to the transport of H-poor knot material by such a flow.
