Dust scattering of emission lines in HII
regions. I. Plane-parallel models and application to the Orion
nebula (M42)
W. J. Henney
Instituto de Astronomía, UNAM, México
Submitted to The Astrophysical Journal, 6 Aug 1997
Abstract:
I present a comprehensive plane-parallel radiative transfer model for
the scattering by dust of emission lines in a blister
HII region on the surface of a molecular cloud. The
effects of dust in the molecular cloud, within the HII
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, semi-analytical and Monte Carlo numerical
techniques are used, as appropriate. The model is applied in detail to
observations of the [OIII] 5007Å line in the inner
Orion nebula. It is shown that the observations are consistent with a
back scattering 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
[OIII] emitting region is of order unity 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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Will Henney <will@astrosmo.unam.mx>
Last modified: Mon Aug 11 17:09:50 Mexico/Verano 1997