Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: an ubiquitous tracer of molecular gas

摘要: We discuss the detection of absorption by interstellar hydrogen fluoride (HF) along sight line to submillimeter continuum sources W49N and W51. have used Herschel's HIFI instrument in dual beam switch mode observe 1232.4762 GHz J = 1-0 HF transition upper sideband band 5a receiver. detected foreground toward both over a wide range velocities. Optically thin components were on lines, allowing us measure - as opposed obtain lower limit column density for first time. As previous observations source G10.6-0.4, derived is typically comparable that water vapor, even though elemental abundance oxygen greater than fluorine four orders magnitude. rather uncertain N(CH) N(H-2) relationship previously diffuse molecular clouds infer exhibiting absorption. Within uncertainties, we find with respect H-2 consistent theoretical prediction main reservoir gas-phase these clouds. Thus, has potential become an excellent tracer hydrogen, provides sensitive probe small density. Indeed, reported here reveal presence low cloud W51 line, at LSR velocity similar 24 km s(-1), had not been identified studies prior launch Herschel.