Intrinsic and Emergent Line Intensities in C13.04
The main output, the file with the code’s predictions and a summary of its behavior, has long included two sets of emission-line intensities, referred to as “Intrinsic” and “Emergent”. In C13 these are described in Hazy 1, section 7.8.6, and Hazy 2, sections 1.10.1 and 1.10.2.
The intrinsic intensities include all locally defined line formation and transfer physics. These intensities account for photo pumping, collisional excitation and deexcitation, line optical depth, and line destruction by “background opacities”, usually grain or photoelectric absorption that occurs where the line forms. This spectrum does not account for extinction that occurs outside of the line-forming region such as foreground dust. The “emergent” intensities account for all this, but also includes line extinction and reflection outside of the region where the lines form. This was originally added to model the optical spectrum of the Orion Nebula. Two spectra are observed. The brighter one is blue shifted and produced in the H+ layer that is moving towards the Earth and away from the background molecular cloud. The albedo of the dust in the molecular cloud is not zero, so a fainter redshifted spectrum is produced by emission from the H+ layer that is directed into the background molecular cloud and is then reflected back towards the Earth. This open geometry, viewed normal to the surface, with a background molecular cloud, was assumed in predicting the emergent intensities through version 13.03.
The fact that this output was designed to predict a particular geometry caused several problems. First, it was originally designed for an open geometry. It was not correct for a dusty closed geometry like an ultracompact H II region. Next, the code reported many integrated quantities that were not transferred lines but rather contains information about the cloud physics. Examples are the heating due to photoionization of hydrogen, or Compton cooling. These “informational” integrated quantities are important since they help to understand why the cloud is the way it is. These were reported in both emergent and intrinsic spectra, although the quantities were the same. With C13.04 we no longer report these in the emergent spectrum, and in a future version will report them in a different section of the printout. Last, all versions through C13 had many lines that were not fully transferred. These included some common forbidden lines. It was not possible to transfer these lines through a complex geometry to predict the emergent spectrum because the needed information was not present. Before C13.04 these lines were reported in both sets of lines, with the same intensity. In C13.04 they are not reported in the emergent intensity. In C16 all lines are fully transferred and are reported in both sets of lines.
We have always recommended correcting the observed spectrum for any external reddening by using a reddening indicator such as the H I line spectrum, and then comparing this dereddened spectrum with the intrinsic line intensities. This remains the best strategy. The emergent intensity is well defined in a closed geometry, but in an open geometry it will depend on the viewing angle. The predictions for an open geometry are for a ray observing the H+ layer along a line orthogonal to its surface.
Gary J. Ferland
2016 July 30
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