THE EXTREME ULTRAVIOLET-SPECTRUM OF G191-B2B AND THE IONIZATION OF THE LOCAL INTERSTELLAR-MEDIUM

An extreme ultraviolet spectrometer launched by a sounding rocket obtained a spectrum of the nearby hot white dwarf G191-B2B from 400 to 1150 Å with ∼15 Å resolution. The stellar effective temperature was measured to be 61,000-4000+6000 K, and the ratio of helium to hydrogen in the stellar photosphere is log (He/H) = -4 ± 0.5, assuming a homogeneous photosphere. Absorption of the extreme ultraviolet stellar spectrum due to neutral hydrogen and helium in the interstellar medium was observed; the neutral helium absorption was detected at the 15 σ level. This observation provided the first direct measurement of the neutral helium to neutral hydrogen ratio in the interstellar medium. The interstellar column densities of neutral helium and hydrogen were found to be NH I = 1.6 ± 0.2 × 1018 cm-2 and NHe I = 9.8-2.6+2.8 × 1016 cm-2. This ratio of neutral hydrogen to neutral helium indicates that the ionization of hydrogen along the line of sight is less than ∼30% unless significant helium ionization is present. The often-cited scenario in which the hydrogen is highly ionized (H II/Htotal ≳ 40%) and the helium is neutral is ruled out by this observation.