The effects of OB stars on the physical state of the interstellar medium (ISM) are studied using a time-dependent numerical simulation which includes explicit two-dimensional hydrodynamics and implicit, coupled radiative transfer and thermodynamics. It is shown that in 10 million years the superposition of the radially expanding H II region shock fronts about OB stars produces a clumped distribution of neutral clouds from an initially homogeneous, isothermal medium. The properties of the observed clouds are best reproduced by an ISM with a hydrogen ionization rate of 10 to the -17th/s and with additional heating supplied by photoelectrons ejected from grain surfaces. Depending on the steady-state equation of state, between 0.1 and 0.7 of the total ISM mass is compacted into clouds which ought to collapse gravitationally.
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Bania et al. (1980) studied this question.