Ultraviolet photons from O and B stars strongly influence the structure and emission spectra of the interstellar medium. The UV photons energetic enough to ionize hydrogen $$h\nu > {\rm 13.6 eV}$$ will create the H II region around the star, but lower energy UV photons escape. These far-UV photons ($$6~{\rm eV} < h\nu < {\rm 13.6~eV}$$) are still energetic enough to photodissociate molecules and to ionize low ionization-potential atoms such as carbon, silicon, and sulfur. They thus create a photodissociation region (PDR) just outside the H II region. In aggregate, these PDRs dominates the heating and cooling of the neutral interstellar medium. The gas is heated by photo-electrons from grains and cools mostly through far-infrared fine structure lines like [O I] and [C II].

The PDR Toolbox is an open-source, science-enabling tool for the community, designed to help astronomers determine the physical parameters of photodissociation regions from observations. Typical observations of both Galactic and extragalactic PDRs come from ground- and space-based millimeter, submillimeter, and far-infrared telescopes such as ALMA, SOFIA, JWST, Spitzer, and Herschel. Given a set of observations of spectral line or continuum intensities, PDR Toolbox can compute best-fit FUV incident intensity and cloud density based on our models of PDR emission. One can also fit H2 rovibrational emission excitation diagrams to determine temperature, column density, and ortho-to-para ratio.

#### Recent Release Highlights

• ###### Updated and expanded Wolfire-Kaufman models

The 2020 WK model set includes updated physics and more CO, H2, and Fe II lines. Newly included are 13CO models.

• ###### KOSMA-tau PDR models

We now include KOSMA-tau PDR models for both clumpy and non-clumpy media. This will provide users the ability to easily compare their observations to models from different PDR codes using the dandy tools in the PDR Toolbox.

• ###### H2 excitation diagram fitting

The H2 excitation fitting tool will create of maps of temperature, column density, and ortho-to-para ratio from observations.

• ###### Improved fitting of observations to models

All fitting now uses the LMFIT package, allowing users to choose the fitting method (e.g., least-squares, MCMC) and giving them more details on quality of fit.

###### Please remember to cite use of the PDR Toolbox!

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