Dynamical expansion of ionization and dissociation fronts around a massive star. I. A mode of triggered star formation

We analyze the dynamical expansion of the H II region and outer photodissociation region (PDR) around a massive star by solving the UV and FUV radiation transfer and the thermal and chemical processes in a time-dependent hydrodynamics code. We focus on the physical structure of the shell swept up by the shock front ( SF) preceding the ionization front ( IF). After the IF reaches the initial Stromgren radius, the SF emerges in front of the IF and a geometrically thin shell bounded by the IF and SF is formed. The gas density inside the shell is about 10(1) - 10(2) times as high as the ambient gas density. Initially, the dissociation fronts (DFs) expands faster than the IF, and the PDR is formed outside the H II region. Thereafter, the IF and SF gradually overtake the preceding DFs, and eventually the DFs are taken into the shell. The chemical composition within the shell is initially atomic, but hydrogen and carbon monoxide molecules are gradually formed. This is partly because the IF and SF overtake the DFs and the SF enters the molecular region, and partly because the reformation timescales of the molecules become shorter than the dynamical timescale. The gas shell becomes dominated by the molecular gas by the time of gravitational fragmentation, which agrees with some recent observations. A simple estimation of the star formation rate in the shell shows that these processes contribute significantly to the star formation rate in our Galaxy.