PDR Model Codes Used in the Toolbox



Wolfire/Kaufman Models

The Wolfire/Kaufman models are primarily based on the work of Tielens & Hollenbach 1985; Wolfire et al. 1990; Hollenbach et al. 1991; Kaufman et al. 1999; and Kaufman et al. 2006. For a given set of gas phase elemental abundances and grain properties, each model is described by a constant H nucleus number density, n, or constant thermal pressure, Pth, and incident far-ultraviolet intensity G0. The models solve for the chemical balance, thermal equilibrium, and radiation transfer through a PDR layer. Our treatment updates previous results to use recent values of atomic and molecular data, current chemical rate coefficients, and grain photolectric heating rates. There are two versions of the "WK" model data sets ("ModelSets") available: 2006 and 2020. We recommend you use the 2020 version as they contain updated physics and chemistry. There are a number model data sets ("ModelSets") currently available in PDR Toolbox. The Wolfire/Kaufman constant density ModelSets present spectral line and continuum intensity ratios as a function of G0 and n. In a future release, we will add constant thermal pressure ModelSets which present the intensity ratios as a function of G0 and thermal pressure Pth. The PDR Toolbox can also support importing the external models of your choice, provided they are stored in the right format.

The ratio plots are used to determine the beam averaged values of G0 and n. 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 values of G0 and n are obtained by fitting the observations to the model space. Required are at least two observations of line intensity ratios or one line intensity ratio and the ratio of the [C II] + [O I] intensity to the infrared dust continuum emission IFIR. More than two observed ratios will of course do a better job of determining unique G0 and n.

It is important to correct for the different beam 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 high temperature region. In such a case, it would not be appropriate to take the ratio of observed fluxes for fitting. Instead 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])/IFIR model ratios are for a single face-on PDR. For the case of active regions of galaxies, or where there may be many clouds contained within 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 model values should be divided by 2).

WK Model Specifics

2020 Models

These model data start with the 2006 PDR model code and add improved physics and chemistry. Critical updates include those discussed in Neufeld & Wolfire 2016, plus photo rates from Heays et al. 2017, oxygen chemistry rates from Kovalenko et al. 2018 and Tran et al. 2018, and carbon chemistry rates from Dagdigian 2019. We have also implemented new collisional excitation rates for [O I] from Lique et al. 2018 (and Lique private communication) and have included 13C chemistry along with the emitted line intensities for [13C II] and 13CO. In Box 1, we show the currently available spectral lines and those we expect to deploy later in the project. In addition, we will compute ModelSets for a full range of metallicity Z.

In pdrtpy, these are referred to as the wk2020 ModelSet.

2006 Models

These models are identical to those available in the "classic" web-based PDR Toolbox. They have metallicity Z=1, and contain [C I], [C II], [O I], Fe, Si, H2 lines and the 12CO ladder up to J=14→13. For some lines, Z=3 is also available.

In pdrtpy, these are referred to as the wk2006 ModelSet.

Spectral lines modeled in PDRT

Current and future spectral line and metallicity coverage of the PDR Toolbox.

WK Model Parameters

Our 2006 models use the standard parameters from Kaufman et al. 1999. Changes for the 2020 models are listed below. The formation rate of H2 goes as Rform nHI n. The PAH abundance XPAH=nPAH/n.

Parameters for 2006 Models
Parameter Symbol Value
Turbulent Doppler velocity δvD 1.5 km s-1
Carbon abundance XC 1.4×10−4
Oxygen abundance XO 3.0×10−4
Silicon abundance XSi 1.7×10−6
Sulfur abundance XS 2.8×10−5
Iron abundance XFe 1.7×10−7
Magnesium abundance XMg 1.1×10−6
Dust abundance relative to diffuse ISM δd 1
FUV dust absorption/visual extinction δUV 1.8
Dust visual extinction per H σV 5×10−22 cm-2
Formation rate of H2 on dust Rform 3×10−17 s-1
PAH abundance XPAH 5×10−7
Cloud H density n 101−107 cm-3
Incident UV flux G0 10−0.5−106.5 Habing
Parameters for 2020 Models Changed from 2006
Parameter Symbol Value
Carbon abundance XC 1.6×10−4
Oxygen abundance XO 3.2×10−4
Dust visual extinction per H σV 5.26×10−22 cm-2
Formation rate of H2 on dust Rform 6×10−17 s-1
PAH abundance XPAH 2×10−7

KOSMA-tau Models

The KOSMA-tau models This KOSMA-tau code was developed from an earlier PDR code, written by A. Sternberg from Tel Aviv University in Israel (Sternberg & Dalgarno 1989; Sternberg & Dalgarno 1995). His original code uses a plane-parallel geometry and was updated to employ spherical geometry (Gierens, Stutzki and Winnewisser 1992; Köster et al. 1994; Störzer, Stutzki and Sternberg 1996; Zielinsky, Stutzki & Störzer 2000).

The KOSMA-tau models come in "clumpy" and "non-clumpy" variety, with 3.1 ≤RV ≤ 5.5 depending on the model. For the non-clumpy models, the mass parameter is the mass of the spherical clump. The density profile and the mass determine the clump radius and the total AV to the clump center. In the clumpy models there are three mass parameters. The main mass parameter in the clumpy models is the total (ensemble) mass of all clumps which are distributed according to a power law with the mass range [Mlow, Mup]. Typically the upper and lower masses are fixed and the total mass is varied.

For more information about these models, see the KOSMA-tau website.

Using Alternate Models

This feature is not fully implemented yet, but it can be used with a little extra work on your part. To be imported in to PDRT, model files must be in FITS format and must follow our given standards. If you would like to try and use alternate models, please contact Marc Pound.

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