Paul F. Goldsmith
The Atomic to Molecular Transition in the Interstellar Medium
Determining time scales is a major challenge for many areas of astrophysics. Star formation is no exception, and the time required for the various steps in the process are critical information for understanding the overall rate of star formation as well as more detailed questions such as the rate of formation of stars of different mass. While much attention has been given to the later stages of star formation, but the earlier steps, notably the formation of molecular clouds, may well be the rate-limiting step, and deserves careful attention. The fundamental definition of a molecular cloud is when its dominant constituent, hydrogen, is predominantly in molecular form. Thus, the long-standing but still imperfectly understood process of molecular hydrogen formation is fundamental. In this talk I will review developments in laboratory measurements of the H2 formation rate. I will also discuss recent work tracing atomic hydrogen in almost-fully molecular clouds, and show how the ratio of atomic to molecular hydrogen can be used to set limits on the age of these regions. I will finally discuss a related aspect of cloud formation, which is the transition of carbon from ionized to neutral, and finally to molecular form, which significantly affects how we trace molecular clouds and determine their mass and evolutionary status.