Extreme-ultraviolet rocket spectroscopy of the hot DA white dwarf G191-B2B

Because of a lack of spectral resolution in existing spectroscopic data, cu rrent white dwarf atmospheric models cannot explain the atmospheric structu re, and resultant extreme-ultraviolet (EUV) emission, of G191-B2B unambiguo usly. Therefore, a new rocket-borne EUV spectrograph was designed, built, c alibrated, and launched. This instrument observed G191-B2B over a bandpass of 254E317 Angstrom and with a resolving power of lambda/Delta lambda appro ximate to 2800. Highly statistically significant absorption features within the flight spectrum, coupled with metal-rich modeled stellar spectra, prov ide evidence for blended features of Fe IV / Fe V and Fe IV / O III in G191 -B2B's atmosphere. A 3 sigma upper limit of 2 x 10(-5) has been placed on t he photospheric helium abundance, arguing against models that rely on He II as a major EUV opacity source.