Spurred by the discovery by the Kuiper Airborne Observatory of widespread, very extended [CII] 157 µm and [OI] 63 µm emission associated with regions of massive star formation, the first models for photodissociation regions (PDRs) appeared thirty years ago. These models described the interaction of far-ultraviolet photons with dense neutral atomic gas separating the highly ionized hydrogen plasma from the surrounding molecular cloud in which these stars are born. Since then we have come to appreciate that PDRs are merely a denser and more energetic example of a widespread phenomena and that essentially the entire neutral interstellar medium of galaxies is a PDR governed by the same physics and chemistry. Indeed, PDR studies cover now surfaces of protoplanetary disks, photo-evaporation of globules and pillars, planetary nebula, characteristics of diffuse interstellar clouds, and the nuclei of galaxies, including starburst and Ultra-Luminous InfraRed Galaxies (ULIRGS) and range from the here and now all the way back through the era of ubiquitous star formation when galaxies were assembled. Clearly, over these 30 years, PDRs have evolved from a curiosity to a mainstay of astronomical research and dense PDRs have become a laboratory for the study of physical and chemical processes relevant for the evolution of the interstellar medium of galaxies. With the results of Herschel Space Observatory and the Atacama Large Millimeter Array now becoming widely available, it is timely to organize a symposium on the many facets of PDRs and their role in studies of the Universe and at the same time honor one of the pioneers of these studies, David Hollenbach.
The goal of this meeting is to overview the state of the art in theoretical PDR studies, to review the processes that control the physical and chemical conditions in PDRs and their emission characteristics, to compare and contrast these models with recent observations of PDRs obtained with the Spitzer Space Telescope, the Herschel Space Observatory, the Stratospheric Observatory For Infrared Astronomy, and the Atacama Large Millimeter Array, to connect studies of dense PDRs in regions of star formation to the studies of the evolution of the interstellar medium of galaxies over the history of the Universe, and to link and compare and contrast studies of PDRs to those of regions dominated by X-rays, by turbulence, by shocks, and by cosmic rays. This symposium aims to bring together astronomers involved in observations of PDRs associated with regions of massive star formation and of the neutral interstellar medium of galaxies, with astronomers versed in the modeling of PDRs and of the infrared spectral characteristics of the interstellar medium, and with atomic and physicists and chemists with a deep understanding of the physical and chemical processes affecting neutral interstellar gas.