A Pre-FUSE View of H₂

Summary

Observations of the interstellar medium within 1 kpc of the Sun with the Copernicus satellite showed a value of the gas to dust ratio that varies by less than a factor of two from its average. The fraction of hydrogen that is molecular is well described by a steady state model that balances formation on grains with photo-destruction. However, in contrast to the local interstellar medium, both in quasar absorption line systems and in circumstellar disks around young stars, there appears to be relatively little H₂. We particularly focus on estimating the amount of H₂ in circumstellar disks around main sequence stars – the environment where planets form.

Introduction

One of the main results achieved with the Copernicus satellite was the systematic measurement of interstellar H and H₂ within about 1 kpc of the Sun. It was found (Savage et al. 1977, Bohlin, Savage & Drake 1978) that the dust to gas ratio is uniform to within a factor of 2 of its average value with the mass in gas being approximately 100 times larger than the mass in dust. Also, the fraction of hydrogen that is molecular, [2N(H₂)]/[N(H) + 2N(H₂)], is well described by a standard steady state model (Hollenbach, Werner & Salpeter 1971). In this standard model, the H₂ is formed on the surface of grains with a rate of about 3 × 10¹⁷ cm³ s⁻¹ (Jura 1975) and destroyed by the absorption of ultraviolet photons with a rate near 5 × 10⁻¹¹ s⁻¹ when the gas is optically thin (Jura 1974).