Comparing gas-phase and grain-catalyzed H2 formation

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.