W. J. Henney, S. J. Arthur, and Ma. T. García-Díaz
ApJ in press (submitted 31 December 2004, revised 13 Mar 2005, accepted 18 Mar 2005)
We present hydrodynamical simulations of the photoevaporation of a cloud with large-scale density gradients, giving rise to an ionized, photoevaporation flow. The flow is found to be approximately steady during the large part of its evolution, during which it can resemble a "champagne flow" or a "globule flow" depending on the curvature of the ionization front. The distance from source to ionization front and the front curvature uniquely determine the structure of the flow, with the curvature depending on the steepness of the lateral density gradient in the neutral cloud.
We compare these simulations with both new and existing observations of the Orion nebula and find that a model with a mildly convex ionization front can reproduce the profiles of emission measure, electron density, and mean line velocity for a variety of emitting ions on scales of 1017-1018cm. The principal failure of our model is that we cannot explain the large observed widths of the [O I] 6300Å line that forms at the ionization front.