PDRs – photodissociation regions – are the technical name for the environment we study most with Mopra. There are molecular clouds in space exposed to interstellar radiation fields. Photo-dissociation is a key process at work – where a far-ultra-violet photon dislodges an electron from a dust grain or molecule, and the kinetic energy of the electron then gets transferred to the gas via collisions. This heats the gas, and also drives a chemistry within. The study of PDRs is the study of the typical environment of the dense interstellar medium – i.e. just what Mopra looks at with its measurements of excited molecules in space!
PDRs have turned 30! For in 1985 a new paradigm was created by the publication of a paper by Xander Tielens and David Hollenbach in the Astrophysical Journal, setting out the basic physics of these regions, and calculating what emission could be observed. The basic paradigm presented then still holds true today, and guides a huge variety of observational programs using infrared and radio telescopes. All research students in astronomy should read it!
A special workshop is being held in Asimolar in California to celebrate 30 years of PDRs, and in particular to recognise David Hollenbach’s lifetime of achievements in science. Xander was David’s first postdoc, and though he lives in Leiden today, he organised this workshop, to which colleagues flocked from around the world. I was a postdoc of David’s too, one of half a dozen of them present. My own time working with David, at NASA Ames Research Center in the Bay Area of California, began 28 years ago, so just 2 years after the famous paper. It was well-known even then, though no-one could have predicted that it would enter astrophysics folk-lore as one of the most important contributions ever made to our understanding of the interstellar medium. I spent much of my time working on the PDR code, in particular considering how emission from molecular hydrogen and carbon monoxide should look like in them. This became a key contribution to science in my own career too.
David Hollenbach discussing how how the famous paper with Xander Tielens was actually written at the workshop. The drawing on the slide shows the key figure from the paper, which presents the structure of photodissociation regions, from the molecular cloud surface irradiated by UV photons. Deeper into the cloud hydrogen atoms transform into hydrogen molecules, and carbon goes from ionized form to neutral form to molecular form (ie CO or carbon monoxide). These are the very species we now study with telescopes like Mopra, Nanten2 and HEAT.