The HRS GTO program to study the neutral hydrogen column density and D/H ratio in the local interstellar medium

Abstract

Early in the HST mission the HRS Team will observe the Lyman alpha line at 100,000 spectral resolution toward 7 late-type local stars. The purpose is to derive the hydrogen and deuterium column densities and D/H ratios along lines of sight toward nearby stars. Here we present theoretical line profiles that demonstrate why 10 5 spectral resolution and high signal-to-noise are needed to derive accurate column densities from spectral lines that lie close to the flat part of the curve of growth and may contain multiple velocity components. The aim of the HRS program is to obtain column densities in the hydrogen and deuterium Lyman alpha lines along a variety of lines of sight within and extending beyond the local cloudlet in which the Sun is located near an edge. The broad chromospheric Lyman alpha emission lines of late-type stars are used as background sources against which to measure the interstellar absorption features. We will also obtain profiles of interstellar absorption features in Mg II and Fe II lines to derive the broadening parameter and/or identify possible multiple velocity components in the lines of sight.