Photoevaporation Flows in Blister H II Regions. I. Smooth Ionization Fronts and Application to the Orion Nebula

• Published in: The Astrophysical journal Vol. 627; no. 2; pp. 813 - 833
• Main Authors: Henney, W. J, Arthur, S. J, García-Díaz, Ma. T
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 10.07.2005; University of Chicago Press
• Subjects: France, Champagne; Ionization front; Orion nebula; hydrodynamics; ISM: individual (Orion Nebula); HII regions; H il regions
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/430593

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 10 super(17)-10 super(18) cm. The principal failure of our model is that we cannot explain the large observed widths of the [O I] l6300 line that form at the ionization front.