Astronomy and Astrophysics – Astrophysics
Scientific paper
2002-11-20
Astronomy and Astrophysics
Astrophysics
16 pages, 4 figures, to be published in the February 2003 issue of the PASP
Scientific paper
10.1086/367546
It has long been known that infrared fine structure lines of abundant ions, like the [O III] 88 micron line, can become optically thick in H II regions under certain high luminosity conditions. This could mitigate their potential as diagnostic tools, especially if the source is too dusty for optical spectroscopy to otherwise determine the system's parameters. We examined a series of photoionization calculations which were designed to push the nebulae into the limit where many IR lines should be quite optically thick. We find that radiative transfer effects do not significantly change the observed emission line spectrum. This is due to a combination of grain absorption of the hydrogen ionizing continuum and the fact that the correction for stimulated emission in these lines is large. Given these results, and the likelihood that real objects have non-thermal line broadening, it seems unlikely that line optical depth presents a problem in using these lines as diagnostics of the physical conditions or chemical composition.
Abel Nicholas
Bryant Adam
Dhakal Prabodh
Ferland Gary
Gale Ashley
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