Comparing Gas‐Phase and Grain‐catalyzed H2Formation

Because H 2 formation on dust-grain surfaces completely dominates gas-phase H 2 formation in local molecular clouds, it is often assumed that gas-phase formation is never important. In fact, it is the dominant mechanism in a number of cases. In this paper, I briefly summarize the chemistry of gas-phase H 2 formation and show that it dominates for dust-to-gas ratios less than a critical value D cr . I also show that D cr is simple to calculate for any given astrophysical situation and illustrate this with a number of examples, ranging from H 2 formation in warm atomic gas in the Milky Way to the formation of protogalaxies at high redshift.