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.

#### New Paper on PDRT!

The PhotoDissociation Region Toolbox: Software and Models for Astrophysical Analysis,
Marc W. Pound & Mark G. Wolfire 2022, AJ, in press. Preprint here.

#### Recent Release Highlights

• ###### H II region ionized gas diagnostic plots

We have developed diagnostic plots for observations of Fe and Ar.

• ###### Expanded testing

More testing added and auto-run at github check-in.

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

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## Tools & more

#### Citing PDR Toolbox

If you use the PDR Toolbox for published research or in a presentation, we ask that you please cite the following references:

#### Support

We gratefully acknowledge support from the NASA Astrophysics Data Analysis Program under award number 80NSSC19K0573; from JWST-ERS program ID 1288 provided through grants from the STScI under NASA contract NAS5-03127; and from the SOFIA C+ Legacy Project through a grant from NASA through award #208378 issued by USRA.

#### History

Back in 2001, when we started the Photodissociation Region Toolbox (PDRT), web programming meant Common Gateway Interface and Perl was King. Single pixel detectors were cutting edge technology and the sub-mm window had just begun to be explored. Airline travel was pleasant and people still smoked cigarettes. We put together PDRT with Perl, HTML, Apache 1.3, FITS files, CVS, shell scripts, thumb tacks, horsehair, and bits of string. The tool filled a need, scratched an itch, developed an international user base. As new telescopes arrived, we added spectral lines and low metallicity models. Web-free scripting interfaces were created by users. Our funding ran out, but we added lines when users requested and kept the service running. Single pixel detectors gave way to pixel arrays and the sub-mm science matured. Recently, we received funding to implement upgrades described here. Let us know which are most important to you and what else you'd like to see!