EXTREME ULTRAVIOLET OBSERVATIONS OF HZ 43 AND THE LOCAL H HE RATIO WITH THE HOPKINS ULTRAVIOLET TELESCOPE/

We present a spectrum of the hot DA white dwarf HZ 43 in the EUV, near the 504 A ionization edge of neutral helium, obtained with the Hopkin s Ultraviolet Telescope (HUT) during the 1990 December Astro-1 mission . The interstellar column densities derived from this spectrum (N(H) = 6.5 +/- 0.5 x 10(17) cm-2; N(He) = 6.6 +/- 1.4 x 10(16) cm-2) rule ou t the anomalous interstellar absorption model proposed by Heise et al. , which required a greater column density of neutral helium than neutr al hydrogen toward HZ 43 in order to explain the low EUV flux from HZ 43 reported by EXOSAT. Instead, we find the interstellar neutral H/He ratio toward HZ 43 to be consistent with the canonical cosmic abundanc e ratio of 10 or with the 11.6 +/- 1.0 ratio measured by HUT along the line of sight toward another DA white dwarf, G191-B2B. The HUT observ ations suggest that either there is a substantial calibration error in the EXOSAT spectroscopy of HZ 43, or otherwise undetected metals in t he nominally pure hydrogen HZ 43 atmosphere suppress its flux between 150 and 300 angstrom, or both.