Vol. 560
In section 6. Interstellar and circumstellar matter

Spatially extended OH+ emission from the Orion Bar and Ridge

The study of the so-called photon-dominated regions or PDRs (in other words, the thin layers separating HII regions from surrounding dense neutral gas) is fundamental for our understanding of how the UV radiation from OB stars affects the environment. The prototype PDR is the "Orion Bar" region 0.5 parsec from the ionizing stars of the Orion Nebula. One of the important discoveries with the Herschel satellite has been that the molecular ions OH+ and H2O+ are good tracers of the hot layers of PDRs, and the article by van der Tak et al. shows that high pressure and probably a high cosmic ray ionization rate are needed to explain the Herschel-HIFI observations of the Bar. This should be important input for efforts to interpret the observation of emission of these species in extragalactic objects such as M82, Arp 220, and Mrk 231.