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With its rich UV-optical-IR spectrum, NGC 7027 has been the target of choice for study of important physical processes including charge-exchange, dust-quenching of nebular lines, PDR's, extended-red emission, etc. Surprisingly little, though, is known about the central star, because the star is embedded in such a bright nebula. With the success of the HST servicing mission, it is now possible to carry out direct studies of the central star. Here, we report on HST/FOS spectroscopy of the central star.
In August 1994, we obtained FOS/G400H, G570H spectra of NGC 7027 with the central star centered in the 1$\farcs$0 aperture. The spectra cover the wavelength range, 3250-6750 \AA. The spectra are of excellent quality. For example, at $\lambda$4800 \AA, the S/N=76 per resolution element, and the star contributes 94\% of the observed continuum flux.
Other very hot stars, i.e. PG 1159 stars, are known to show high-ionization lines, such as O VI $\lambda$$\lambda$3811,3834, C IV $\lambda$4441, or C V $\lambda$4945, etc. To our surprise, these lines are absent in the spectrum of the central star. In fact, the stellar spectrum is apparently featureless. We have therefore used the absence of spectral features to put limits on effective temperature and chemical composition of the central star. We are constructing a small grid of line-blanketed NLTE model atmospheres for the effective temperatures range $100,000 \, {\rm K} < T_{\rm eff} < 250,000 \, {\rm K}$, $5.8 < \log g < 7.5$, and for two compositions: $(i)$ a H-rich (solar) composition, and $(ii)$ a C-O enriched atmosphere, typical of PG 1159 stars. We will report on the results of this analysis.
