The HF molecule is an excellent tracer of H2, even at low column densities where CO is photodissociated, as shown by widespread absorption in the 1--0 line at 1232 GHz seen with Herschel. We have made the surprising observation that this line appears in emission towards the Orion Bar, which is where ultraviolet radiation from the Trapezium stars hits the molecular cloud and causes heating and dissociation. Three mechanisms may cause this unusual effect: thermal excitation, which requires very high gas densities; radiative pumping by dust continuum or H2 line emission at ~2.5 microns; or chemical pumping where most HF is formed in the J=1 state. These models each predict a distinct spatial distribution of HF, which is why we hereby propose one-dimensional maps of HF in the Orion Bar and two similar regions. Combined with existing maps of CO and dust emission, the new data will constrain the density structure of molecular clouds in the low column density regime which is inaccessible to other telescopes, and thus offer fundamental insight into the physics of the interstellar medium.
Origin of hydrogen fluoride emission in the Orion Bar. An excellent tracer for CO-dark H2<-> gas clouds . Kavak Ãœ. et al. . Astronomy and Astrophysics . null . null . 2019A&A...631A.117K ,
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.