Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction

摘要

We report the detection of absorption by interstellar hydroxylcations and water cations, along the sight-line to the bright continuumsource W49N. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 972GHz N=1-0 transition of and the 1115GHz 111-000 transition of ortho-.The resultant spectra show absorption by ortho-,and strong absorption by OH+, in foreground material at velocities in the range 0 to 70with respect to the local standard of rest. The inferred abundance ratio ranges from 3 to 15, implying that the observed OH+ arises in clouds of small molecular fraction, in the range. This conclusion is confirmed by the distribution of OH+ and H2O+in Doppler velocity space, which is similar to that of atomic hydrogen,as observed by means of 21cm absorption measurements, anddissimilar from that typical of other molecular tracers. The observed abundance ratio of a few suggests a cosmic ray ionization rate for atomic hydrogen of ,in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H3+ and other species. Key words: ISM:molecules - submillimeter:ISM - astrochemistry - molecular processes