Astrophysics Luncheon Seminar Origin of Hydrogen Fluoride (HF) Emission in the Orion Bar Presented by Ümit Kavak University of Groningen Monday, July 9, 2018 12:00 noon in 169-336 and on Webex Abstract The hydrogen fluoride (HF) molecule is seen in absorption in the interstellar medium (ISM) along with many lines of sight. It is surprisingly observed in emission toward the Orion Bar, which is an interface between the ionized region around the Orion Trapezium stars and the Orion Molecular Cloud. The aim of this talk is to understand the origin of HF emission in the Orion Bar by comparing its spatial distribution with other tracers. We consider three mechanisms: thermal excitation, radiative dust pumping, and chemical pumping which require distinct spatial distribution. We use a Herschel-HIFI map of the HF 1-0 line of 0.50’ by 1.50’ oriented perpendicular to the Orion Bar. We use RADEX, non-local thermodynamic equilibrium (non-LTE) code to construct the HF column density map. The spectral energy distribution (SED) of three positions within the HF map is constructed to investigate the dust contribution. The bulk of the HF emission at 10 km/s emerges between the ionization front and the dense gas in the Orion Bar. We find that HF is excited in collisions with H2 and free electrons; dust pumping and chemical pumping are less important. Additional HF emission is observed at 7 km/s toward the HII region, which arises in molecular gas in the background. A weak absorption component is detected at 5 km/s which has an HI counterpart and originates in the Orion Veil in the foreground. The excitation of Galactic HF is caused mostly by dense gas with a high electron abundance in the Orion Bar. We concluded that HF emission traces CO-dark molecular gas as H2 has to be abundant to make HF and electrons (i.e., C+) to provide the excitation. Similar conditions may help to explain extra-galactic HF emission. JPL Contact: Paul Goldsmith (3-0518) About the Speaker Ümit is a second year Ph.D. student at the University of Groningen and SRON Netherlands Institute for Space Research. He is doing his thesis with Floris van der Tak and Xander Tielens. For his Ph.D. thesis, he is aiming to understand the link between molecular clouds and massive young stars, using data from the Herschel Space Observatory and the Stratospheric Terahertz Observatory. Before that, he completed his MSc at the University of Cologne under the supervision of Peter Schilke and Alvaro Sanchez-Monge, where he searched for radio jets in massive star-forming regions with the VLA.   Site Manager: Webmaster: CL 08-3220