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).