Extreme-ultraviolet spectrophotometry of the hot DA white dwarf HZ 43: detection of He II in the stellar atmosphere

ae report a spectrum of the hot white dwarf HZ 43 in the extreme ultraviolet band from 170--500 A with a resolution of 15 A. The spectrum is marked by a prominent absorption edge at 225 A +- 15 A with N/sub lambda/ (220 A)/N(236 A) = 0.51 +- 0.13. The observed edge is attributed to absorption at the 228 A Lyman edge of singly ionized helium. A grid of blanketed LTE model atmospheres with uniform compositions of hydrogen with trace helium were used to interpret the data. We find that we are able to obtain satisfactory fits to our data for effective temperatures in the range 45,000--60,000 K either for pure hydrogen models with an interstellar He II column density in the range (2.8--6) x 1017 ions cm-2, or for models with a photospheric helium fractional number densities in the range (1.5-6) x 10-5 of hydrogen. Our results, combined with observations at other wavelengths, yield a best estimate of 55,000--60,000 K for the effective temperature. We derive an upper limit on the interstellar column densities of N(H)< or =2 x 1017 cm-2 for neutral hydrogen, of N(He I)< or =1017 cm-2 for neutral helium and of N(HeII)< or =6 x 1017 cm for singly ionized helium. However, no single pure hydrogen or uniform trace helium model will simultaneously fit the available optical to soft X-ray data. We have evaluated possible sources of He II in the interstellar medium and conclude that they are unlikely to contribute the entire observed column density. Hence the observations provide the first detection of helium in HZ 43