We present the results of spectroscopy of four bright proplyds in the Orion Nebula obtained at a velocity resolution of 6 km/s. After careful isolation of the proplyd spectra from the confusing nebular radiation, the emission line profiles are compared with those predicted by realistic dynamic/photoionization models of the objects. The spectral line widths show a clear correlation with ionization potential, which is consistent with the free expansion of a transonic, ionization-stratified, photoevaporating flow. Fitting models of such a flow simultaneously to our spectra and HST emission line imaging provides direct measurements of the proplyd size, ionized density and outflow velocity. These measurements confirm that the ionization front in the proplyds is approximately D-critical and provide the most accurate and robust estimate to date of the proplyd mass loss rate. Values of 0.7E-6 to 1.5E-6 Msun/year are found for our spectroscopic sample, although extrapolating our results to a larger sample of proplyds implies that 0.4E-6 Msun/year is more typical of the proplyds as a whole. In view of the reported limits on the masses of the circumstellar disks within the proplyds, the length of time that they can have been exposed to ionizing radiation should not greatly exceed 10,000 years - a factor of 30 less than the mean age of the proplyd stars. We review the various mechanisms that have been proposed to explain this situation, and conclude that none can plausibly work unless the disk masses are revised upwards by a substantial amount.Comment: 23 pages, 8 figures, uses emulateapj.sty, accepted for publication in The Astronomical Journal (scheduled November 1999

This is a summary of the 2013 release of the plasma simulation code Cloudy. Cloudy models the ionization, chemical, and thermal state of material that may be exposed to an external radiation field or other source of heating, and predicts observables such as emission and absorption spectra. It works in terms of elementary processes, so is not limited to any particular temperature or density regime. This paper summarizes advances made since the last major review in 1998. Much of the recent development has emphasized dusty molecular environments, improvements to the ionization / chemistry solvers, and how atomic and molecular data are used. We present two types of simulations to demonstrate the capability of the code. We consider a molecular cloud irradiated by an X-ray source such as an Active Nucleus and show how treating EUV recombination lines and the full SED affects the observed spectrum. A second example illustrates the very wide range of particle and radiation density that can be considered.

We present the results of radiation-magnetohydrodynamic simulations of the formation and expansion of H ii regions and their surrounding photodissoci

We present numerical radiation-hydrodynamic simulations of the evolution of HII regions formed in an inhomogeneous medium resulting from turbulence simulations. We find that the filamentary structure of the underlying density distribution produces a highly irregular shape for the ionized region, in which the ionization front escapes to large distances in some directions within 80,000 years. In other directions, on the other hand, neutral gas in the form of dense globules persists within 1 parsec of the central star for the full duration of our simulation (400,000 years). Divergent photoablation flows from these globules maintain a root-mean-squared velocity in the ionized gas that is close to the ionized sound speed. Simulated images in optical emission lines show morphologies that are in strikingly detailed agreement with those observed in real HII regions.Comment: Minor changes to sync with accepted version. 7 pages, ApJ in press. Accompanying video available at http://ifront.org/wiki/Turbulent_Hii_Regions/Paper

We investigate whether simple models of a photoevaporated Ñow from an externally ionized neutral clump or wind can reproduce the observed Ha intensity proÐles of the proplyds in the inner Orion Nebula. We Ðnd that models Ðtted to the bright "" cusp ÏÏ at the head of each proplyd successfully predict the brightness distribution of the extended di †use "" atmosphere ÏÏ between the proplyd and the ionizing star of the nebula (h1 Ori C). This is strong evidence that the "" atmospheres ÏÏ are not conÐned but are freely expanding photoevaporated Ñows, with implied mass-loss rates of up to 10~7 yr~1. The only M _ 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 Ñattened disk. The model Ðts imply that the initial Mach number of the Ñow from the ionization front is close to unity, which is consistent with theoretical predictions that such fronts should be approximately Dcritical. The dust-togas ratio in the Ñow 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 h1 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 h1 Ori C. Both these statements are supported by the distribution of inclination angles of the best-Ðt proplyd models, which allow the three-dimensional distribution of proplyds to be constrained.