THE DISRUPTION OF MOLECULAR CLOUD CORES BY PHOTOIONIZATION

We solve the line transfer problem in evolving H Ir regions in order t o calculate line profiles of hydrogen recombination (H alpha) and forb idden oxygen ([O III] lambda 5007) lines along several lines of sight during the photodisruption of molecular cloud cores, or high-density c ondensation. The density, velocity, and ionization structure of spheri cally symmetric models with an initial power-law density distribution, rho proportional to r(-w), were used to calculate the source function . We differentiate between two possible evolutions: the classical evol ution (w less than or equal to 1.5), in which upon expansion of the io nized gas a shock is driven into the neutral intercloud medium, and ev olution for steeper density gradients (w > 1.5), in which the ''champa gne'' phase develops as the whole cloud becomes ionized by a supersoni c R-type ionization front. Thus a strong shock is driven into the ioni zed gas by the expansion of the denser cloud core. The rapid expansion of these high-density cores generates supersonic outflows as well as important variations in the H II equilibrium temperature, which ranges from 10(3) K within the core to 8 x 10(4) K behind the champagne shoc ks. As a result, the line profiles in these cases may present partial or total splitting both in H alpha and in [O III] lambda 5007. Also th e surface brightness distributions of the oxygen line traces mainly th e hot (T > 3 x 10(4) K) and fast-moving shocked gas, and the H alpha t races the slower, purely photionized matter (T similar to 10(4) K). Th us the continuous and rapid disruption of condensations, driven by the pressure imbalance created by photoionization within a star-forming c loud, adds a supersonic bulk motion to the uniform velocity field expe cted from the classical evolution.