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Another way to estimate the density is to measure the

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Unformatted text preview: low there may not be an equilibrium solution. This is generally not the case in a PDR, but in the diffuse ISM such situations arise and result in runaway heating (until collisional excitation of optical lines takes over). This will be discussed in the lecture on the warm neutral medium. A final aspect of the problem is that the [O I] line may develop optical depths of order unity in a PDR (column NH > 1021 cm−2). We will not discuss the radiative transfer aspects of the problem here. 4. Diagnostics We may finally discuss the key diagnostics for PDRs. These have many similarities to the diagnostics for H II regions, but we work at a lower energy and 7 dust plays a critical role. As always, we must watch out for inhomogeneities – real PDRs are not idealized spherical shells. A. INCIDENT FLUX The incident UV flux (or G0) may be determined by the calorimetry method: the radiation is reprocessed into the FIR, and one may simply take the FIR surface brightness as an estimate of the UV flux. This method requires assumptions about the 3D geometry, which may not be independently known. An alternative method is to use observations of the central star(s) and use our knowledge of stellar spectra and luminosities as a function of spectral type. A final method is to use the H2 fluorescence lines, which are pumped by the FUV continuum. Essentially all incident FUV photons within the spectral lines will be repr...
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This document was uploaded on 03/08/2014 for the course AY 102 at Caltech.

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