Models of gas-grain chemistry in star-forming regions

It is difficult if not impossible to explain the abundances of assorted interstellar molecules in both the gaseous and condensed phases without the use of grain chemistry. Unfortunately, the chemistry occurring on grains is not well understood because of a variety of uncertainties including the nature, size, and shape of dust particles, the binding energies of key species, the dominant mechanism of surface chemistry, and the correct mathematical treatment of surface processes. Still, intrepid astrochemists have used granular chemistry in chemical models of an assortment of sources including cold clouds, protostellar disks, and hot cores. Indeed, the dominant explanation of the saturated gas-phase molecules observed in hot cores involves grain chemistry during an earlier, low temperature phase. Although gas-grain models have elucidated major features of the chemistry, much more work remains to be accomplished before they can be used to help characterize the physical conditions in star-forming regions and their temporal variations.