2001ApJ...562..992G

Because of a lack of spectral resolution in existing spectroscopic data, current white dwarf atmospheric models cannot explain the atmospheric structure, and resultant extreme-ultraviolet (EUV) emission, of G191-B2B unambiguously. Therefore, a new rocket-borne EUV spectrograph was designed, built, calibrated, and launched. This instrument observed G191-B2B over a bandpass of 254-317 Å and with a resolving power of λ/Δλ~2800. Highly statistically significant absorption features within the flight spectrum, coupled with metal-rich modeled stellar spectra, provide evidence for blended features of Fe IV/Fe V and Fe IV/O III in G191-B2B's atmosphere. A 3 σ upper limit of 2x10^–5 has been placed on the photospheric helium abundance, arguing against models that rely on He II as a major EUV opacity source.