H₂ Absorption Line Measurements with ORFEUS

Summary

We review recent H₂ absorption line measurements in the diffuse interstellar medium, using FUV spectra from the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS). We investigate molecular hydrogen gas along lines of sight toward 5 stars in the Magellanic Clouds and toward 3 stars within the Milky Way. Molecular fractions in gas within the Magellanic Clouds are significantly lower than typically found in gas in the Milky Way, most likely caused by the lower dust content. The finding of H₂ in a Galactic high-velocity cloud led us to speculate that the high-velocity gas in front of the Magellanic Clouds is part of the Galactic fountain. Sight lines toward the Galactic stars show well defined absorption by molecular hydrogen, deuterium and metals, allowing the study of physical and chemical conditions in the local interstellar gas in great detail.

Introduction

Molecular hydrogen is by far the most abundant molecule in the interstellar medium. Its measurement, however, is difficult: H₂ has no permanent dipole moment and no radio emission is seen from H₂, in striking contrast to the second most abundant molecule in the ISM, carbon monoxide (CO). For the study of the diffuse interstellar medium the FUV absorption spectroscopy is the only method to obtain information about the molecular hydrogen content, but satellites are required for this method, since the earth's atmosphere is opaque for radiation in the FUV domain.