Observations of the local interstellar medium with the Hopkins Ultraviolet Telescope

Abstract

During the Astro‐1 space shuttle mission of December 1990, the Hopkins Ultraviolet Telescope was used to carry out an absorption study of the local interstellar medium (LISM). Through EUV observations of the hot DA white dwarf stars G191‐B2B and HZ43, neutral hydrogen and neutral helium column densities have been determined along two lines of sight through the local interstellar cloud surrounding the Sun. The neutral hydrogen to helium ratios observed (in comparison to the cosmic abundance ratio of the two elements) provide an assessment of the relative ionization of the two species in the LISM. We find, in contrast to some previous indirect determinations, that hydrogen is not preferentially ionized compared with helium in the LISM and thus, that exotic sources of ionization are not required to explain the ionization state of the local cloud.