PDR Model Details

The PDR models used for these tools are described in Kaufman, Wolfire, Hollenbach, & Luhman 1999. The models are based on the work of Tielens & Hollenbach 1985; Wolfire, Tielens, & Hollenbach 1990; and Hollenbach, Takahashi, & Tielens 1991. For a given set of gas phase elemental abundances and grain properties, each model is described by a constant H nucleus density, n, and incident far-ultraviolet intensity G₀. The models solve for the equilibrium chemistry, thermal balance, and radiation transfer through a PDR layer. The Kaufman et al. treatment updates the previous results to use recent values of atomic and molecular data, current chemical rate coefficients, and grain photolectric heating rates which include treatments of small grains and large molecules.

References

Hollenbach, D. J., Takahashi, T., & Tielens, A. G. G. M. 1991, ApJ, 377, 192.
Kaufman, M. J., Wolfire, M. G., Hollenbach, D. J., & Luhman, M. L. 1999, ApJ, 527, 795.
Tielens, A. G. G. M., & Hollenbach, D. J. 1985, ApJ, 291, 722.
Wolfire, M. G., Tielens, A. G. G. M., & Hollenbach, D. J. 1990, ApJ, 358, 116.

Guide for Using PDR Contour Plots

The contour plots are used to determine the beam averaged PDR gas density, n, and incident far-ultraviolet (FUV; 6 eV < E < 13.6 eV) radiation field G₀ . Note that n is the H nucleus density (cm^-3) so that the maximum abundance of H₂, for example, is n(H₂)= 0.5 n. The radiation field is measured in units of the local Galactic interstellar FUV field found by Habing (= 1.6 x 10^-3 erg cm^-2 s^-1). The values of n, and G₀ are obtained by overlaying two sets of contour plots and finding the intersection of the observations. Required are either (i) two sets of line intensity ratios or (ii) one set of line intensity ratios and the ratio of the [C II] + [O I] intensity to the infrared dust continuum emission. We have also provided a diagnostic plot combining the [C II]/[O I] and ([C II]+[O I])/FIR ratios as in Wolfire, Tielens, & Hollenbach 1990.

Note that it is important to correct for the different area filling factors for various species. For example, diffuse [C II] emission may fill the observing beam whereas [O I] emission may arise from a smaller, high density and temperature region. In such a case it would not be appropriate to take the ratio of observed fluxes for use in the contour plots. The ratio of intensities should be used or the ratio of fluxes corrected for the separate emitting solid angles. Note also that the ([C II] + [O I])/FIR contours are for a single face-on PDR. For the case of active regions of galaxies, or where many clouds are contained with the beam, the FIR continuum is observed from both the near and far side of the cloud. Thus, the observed ratio should be multiplied by a factor 2 (or equivalently the contour values should be divided by 2) before using the contour plot. See additional information and examples in Kaufman et al. (1999).

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