Extreme Ultraviolet Observations of HZ 43 and the Local H/He Ratio with the Hopkins Ultraviolet Telescope

Abstract

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 Hopkins Ultraviolet Telescope (HUT) during the 1990 December Astro-1 mission. The interstellar column densities derived from this spectrum rule out the anomalous interstellar absorption model proposed by Heise et al.(1991), which required a greater column density of neutral helium than neutral 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 abundance 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 observations suggest that either there is a substantial calibration error in the EXOSAT spectroscopy of HZ 43, or otherwise undetected metals in the nominally pure hydrogen HZ 43 atmosphere suppress its flux between 150 and 300 A, or both.