Abstract
The main properties of the evolution of H II regions in clouds with disk-like density distributions are derived analytically. The clouds are asummed cylindrically symmetric with exponential, gaussian, and sech2 density stratifications. The dimensions of the initial H II regions along each angle, 0, are described in terms of the Stromgren radius for the mid plane density, R0, and the disk scale heigth, H. For ν0 = R0 sin(θ)/H < α (where α is a constant dependent on the assumed density distribution) the whole H II region is contained within the disk, and for y0 > α a conical section of the disk becomes totally ionized. The critical azimuthal angle above which the H II region becomes unbounded is defined by θcrit = sin^-1(αH/R0). The expansion of initially unbounded H II regions (i.e. with y0 > α), as discussed by Franco et at. 1989, proceeds along z-axis and, if the disk column density remains constant during the evolution, the ionization front eventually recedes from infinity to become trapped within the expanding disk. For clouds by a B-field oriented parallel to the symmetry axis, as expected in magnetically dominated clouds, this effect can be very prominent. The expanding gas overtaken by the receding ionization front maintains its linear momentum after recombination and is transformed into high-velocity neutral outflow. In the absence of magnetic fields, the trapping has only a short duration. Further particular aspects of the these receding ionization fronts are studied with detailed hydrodynamical numerical simulations of photoionized regions. These show the existence of shocks in the neutral flows and provide more details of the trapping stage.
