DYNAMICAL EXPANSION OF IONIZATION AND DISSOCIATION FRONT AROUND A MASSIVE STAR. II. ON THE GENERALITY OF TRIGGERED STAR FORMATION

摘要

We analyze the dynamical expansion of the H Ⅱ region, photodissociation region, and the swept-up shell, solving the UV and far-UV radiative transfer and the thermal and chemical processes in the time-dependent hydrodynamics code. Following our previous paper, we investigate the time evolutions with various ambient number densities and central stars. Our calculations show that basic evolution is qualitatively similar among our models with different parameters. The molecular gas is finally accumulated in the shell, and the gravitational fragmentation of the shell is generally expected. The quantitative differences among models are well understood with analytic scaling relations. The detailed physical and chemical structure of the shell is mainly determined by the incident far-UV flux and the column density of the shell, which also follow the scaling relations. The time of shell fragmentation and the mass of the gathered molecular gas are sensitive to the ambient number density. In the case of a low density, the shell fragmentation occurs over a longer timescale, and the accumulated molecular gas is more massive than in the case of a high density. The variations with different central stars are more moderate. The time of the shell fragmentation differs by a factor of several with the various stars of M_* = 12-101 solar mass. According to our numerical results, we conclude that the expanding H Ⅱ region should be an efficient trigger for star formation in molecular clouds if the mass of the ambient molecular material is large enough.