Modeling the Brightness Profiles of the Orion Proplyds

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Abstract
We investigate whether simple models of a photoevaporated flow from an externally ionized neutral clump or wind can reproduce the observed Hα intensity profiles of the proplyds in the inner Orion Nebula. We find that models fitted to the bright "cusp" at the head of each proplyd successfully predict the brightness distribution of the extended diffuse "atmosphere" between the proplyd and the ionizing star of the nebula (θ1 Ori C). This is strong evidence that the "atmospheres" are not confined but are freely expanding photoevaporated flows, with implied mass-loss rates of up to 10-7 M⊙ yr-1. The only way to reconcile such high mass-loss rates with the age of the nebula, the high fraction of stars that are proplyds, and the low extinction to these stars is for the neutral reservoir to be in the form of a flattened disk. The model fits imply that the initial Mach number of the flow from the ionization front is close to unity, which is consistent with theoretical predictions that such fronts should be approximately D-critical. The dust-to-gas ratio in the flow is not tightly constrained by the models. The distribution of projected radii of sources in the inner Trapezium cluster indicates that (1) both the proplyds and the nonproplyd stars in the cluster are strongly peaked about θ1 Ori C on a 0.02 pc scale, much smaller than has previously been suggested, and (2) the fraction of low-mass stars that are proplyds probably decreases smoothly with increasing physical distance from θ1 Ori C. Both these statements are supported by the distribution of inclination angles of the best-fit proplyd models, which allow the three-dimensional distribution of proplyds to be constrained.