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Session 41 - T Tauri Stars & Protostellar Regions.
Display session, Wednesday, June 11
South Main Hall,
Observations of HII regions show that the bulk of the emission comes from a thin and very highly stratified region at the interface between the ionized gas and the molecular cloud. Based on WFPC2 images of the elephant trunks in M16, we have shown (Hester et al. 1996, AJ, 111, 2349) that this interface can be understood as a photoionized, photoevaporative flow. We also argue that this same basic understanding can be applied to the bulk of emission from most HII regions seen at visible wavelengths.
An interface in which the stellar continuum responsible for driving the photoevaporative flow is also responsible for ionizing that flow represents a well-defined physical structure. All aspects of the emission are determined by a suite of parameters including the shape and intensity of the ionizing continuum, the density of the molecular cloud, and the abundances in the gas. Such a framework could in principle guide the interpretation of HII region spectra, replacing the assumptions usually made with an understanding paralleling that of models of radiative shocks.
We have begun to study the structure of such interfaces. Here we present initial results of model calculations of the emission from photoevaporative flows which include photoionization and hydrodynamics.
