THE DEVELOPMENT OF H-I DISSOCIATION ZONES AROUND NEW H-II REGIONS

We present the results of computations for a model of the time development of H I photodissociation zones in the molecular gas around O and early B main-sequence stars and their associated H II regions. The computations are for a grid of values of stellar effective temperatures (T(eff)) from 20 to 45 kK (spectral types B2.5 V-O5 V) and of ambient gas particle densities from 30 to 3000 atoms cm-3. We follow the conditions in the atomic gas in and behind the advancing dissociation front from an assumed stellar "switch-on" time until the ionization and associated shock front overtakes the zone of dissociation. A fraction of H-2 molecules which absorb photons in the Lyman-Werner bands are dissociated during the resulting electronic and vibrational excitation. The dissociation rates are much higher in regions close to the star where the UV energy density is sufficient to provide re-excitation before the molecules decay to the vibrational ground state. For most combinations of stellar type and gas density this ensures an initial rapid formation of an H I zone. The dissociation front then advances into the molecular gas on the time scale of the expansion of the H II region. As the H II region expands, the surrounding H I zone is eventually eroded by the advancing shock and ionization fronts. The model calculations show the following features of atomic zones around H II regions: In low-density gas the unshocked H I zone will persist for up to half the main-sequence lifetime of the star, whereas in high-density gas the zone will last only a few percent of the stellar lifetime. A star with a T(eff) at the lower end of the range will dissociate to a radius several times the radius of the H II region, whereas the maximum width of the dissociation zone around a star at the early end of the range will be less than the ionized radius. The sizes and masses of both H II regions and their H I zones increase with decreasing density of the surrounding gas. For a given gas density, the total mass of atomic gas formed is not nearly as strong a function of stellar type as is the total mass of ionized gas. For a T(eff) of 45 kK, the mass of H I at the time when the shock overtakes the dissociation front ranges from 25 M. in gas of density 3000 cm-3 to 10(5) M. in gas of density 30 cm-3. The corresponding range for a T(eff) of 20 kK is from 20 M. to 2 x 10(4) M.. Although dissociation zones will form around all H II regions, they will be relatively more prominent where the exciting star has an effective temperature below 30,000 K. One can expect to detect photodissociated H I around young systems with exciting stars earlier than about spectral type B4. Several heating and cooling processes are included in the model. The dominant heating processes are the dissociation itself in the advancing front, and the photoelectric effect on dust grains elsewhere. The most prominent cooling processes are infrared lines from H-2 molecules in molecular gas near the front, and the C+ line at 158-mu-m in the dissociated atomic gas. Gas temperatures near young, rapidly advancing dissociation fronts may be as high as 1000 K, but values near 100 K are more typical of developed H I zones. We discuss the detectability of H I zones. In about seven cases where sufficiently detailed observations are available, we compare the results of the model calculations with observations of 21 cm emission and absorption from H I zones surrounding H II regions. The exciting stars in these observations span the range from B5 to O6. The modeling calculations are generally in accord with the observations and permit a more comprehensive interpretation.