Dust Scattering of Emission Lines in HiiRegions. I. Plane‐parallel Models and Application to the Orion Nebula (M42)

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Abstract
I present a comprehensive plane-parallel radiative transfer model for the scattering by dust of emission lines in a blister H II region on the surface of a molecular cloud. The effects of dust in the molecular cloud, within the H II region, and in a layer of overlying neutral material are all included. The spectral intensity and polarization profiles of the scattered lines are calculated, as well as the effective albedo of the scattering. Analytical, semianalytical and Monte Carlo numerical techniques are used, as appropriate. The model is applied in detail to observations of the [O III] λ5007 Å line in the inner Orion Nebula. It is shown that the observations are consistent with a backscattering origin for the broad redshifted component that is seen in this line, as well as in lines of other ionized species. High-resolution spectropolarimetry of the emission line would provide a conclusive test of the scattering model. Model fits suggest that the optical depth of dust between the ionization front and the [O III] emitting region is in the range 0.5-1.0 in the visible. Foreground scattering in the neutral lid that overlies the nebula is shown to have little effect on the line profile, although it could account for some of the unexplained broadening seen in all optical lines.

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