Dynamical Formation of Dark Molecular Hydrogen Clouds around Diffuse HiiRegions

We examine the triggering process of molecular cloud formation around diffuseHII regions. We calculate the time evolution of the shell as well as of the HIIregion in a two-phase neutral medium, solving the UV and FUV radiativetransfer, the thermal and chemical processes in the time-dependenthydrodynamics code. In the cold neutral medium, the ambient gas is swept up inthe cold (T \sim 30-40K) and dense (n \sim 10^3 cm^-3) shell around the HIIregion. In the shell, H_2 molecules are formed from the swept-up HI gas, but COmolecules are hardly formed. The reformation of H_2 molecules is more efficientwith a higher-mass central star. The physical and chemical properties of gas inthe shell are just intermediate between those of the neutral medium andmolecular clouds observed by the CO emission. The dense shell with cold HI/H_2gas easily becomes gravitationally unstable, and breaks up into small clouds.The cooling layer just behind the shock front also suffers from thermalinstability, and will fragment into cloudlets with some translational motions.We suggest that the predicted cold ``dark'' HI/H_2 gas should be detected asthe HI self-absorption (HISA) feature. We have sought such features in recentobservational data, and found shell-like HISA features around the giant HIIregions, W4 and W5. The shell-like HISA feature shows good spatial correlationwith dust emission, but poor correlation with CO emission. Our quantitativeanalysis shows that the HISA cloud can be as cold as T \sim a few x 10K.(abridged)