On the possibly low H2 formation rate in dense clouds

It has been found for the Rho Ophiuchi cloud, and suggested for diffuse cloud cores in general, that H2 has a lower formation rate than normally expected in diffuse clouds. It is proposed that this may be due to a reduction in grain surface area per unit volume, if the grains in these regions are enlarged due to coagulation. For the Rho Oph cloud, there is independent evidence that grain coagulation has occurred, so this hypothesis regarding H2 formation has a more substantial basis than for other clouds, where the evidence for suppressed H2 formation is less certain, and for which there is little information on grain sizes.