Dynamical formation of dark molecular hydrogen clouds around diffuse HII regions

We examine the process that triggers molecular cloud formation around diffuse H II regions. We calculate the time evolution of the shell, as well as the H II region, in a two-phase neutral medium, solving the UV and FUV radiative transfer and the thermal and chemical processes with a time-dependent hydrodynamics code. In the cold neutral medium, the ambient gas is swept up in a cold (T similar to 30-40 K) and dense (n similar to 103 cm similar to 3) shell around the H II region. In the shell, H-2 molecules are formed from the swept-up H I gas, but hardly any CO is formed. This is a result of the different efficiencies of self- shielding effects between H-2 and CO molecules. The physical and chemical properties of gas in the shell are just intermediate between those of the neutral medium and molecular clouds observed in CO emission. We suggest that the predicted cold "dark'' H1/H2 gas should be detectable as an H I self-absorption (HISA) feature. We have sought such features in recent observational data and found shell-like HISA features around the giant H II regions W4 and W5. These features shows good spatial correlation with dust emission, but poor correlation with CO emission. Our quantitative analysis shows that the HISA cloud can be as cold as a few tens of kelvins. In the warm neutral medium, on the other hand, an expanding diffuse H ii region is much simpler, owing to a small pressure excess. The UV photons only ionize the neutral medium and produce a warm ionized medium.