[Previous] | [Session 28] | [Next]
B.D. Moore (Rice University), J.J. Hester (Arizona State University)
Narrow-band images of two wind-blown nebulae NGC~6888 and NGC~7635 taken with the Hubble Space Telescope are presented. The line emission from each object is dominated by structure on scales finer than the limits imposed on images taken from the ground. The high spatial resolution permits not only separation of shock-heated and photoionized zones, but also the ionization structure within them. Models of realistic density profiles were employed within the photoionization code CLOUDY to match the observed structure.
NGC~6888 is the wind-blown nebula formed as a fast wind from the Wolf-Rayet star HD~192163 swept up material shed during the star's previous red supergiant phase. The abundances of nitrogen, oxygen, and sulfur for models of several ionized knots are significantly different than those derived from analysis of ground-based spectroscopy. This is in part due to the use of ionization correction factors (iCFs) to interpret the spectra.
NGC~7635 is the shell formed by the stellar wind interaction with the material in the surrounding \ion{H}{2} region. Models of two structures were constructed with density profiles approximating photoevaporative flows. The abundances used in the models were consistent with those derived from spectra. The apparent magnitude of the model star matches observations, but its effective temperature disagrees with that implied by its spectral type.
Motivated by these results, the general effect of structure on the emission-line spectrum of a nebula and its interpretation was investigated. Photoionization models of \ion{H}{2} regions based on realistic density distributions show large discrepancies compared to those from constant density models. Analysis of the integrated emission of these models and those of discrete knots exhibit even larger discrepancies. This indicates that small-scale structure invalidates many of the assumptions underlying the iCF methodology, leaving its general use suspect.
This work was supported at Arizona State University by NASA/JPL contracts 95-9289 and 95-9329 and Caltech contract PC 064528.