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

Abstract

We report the detection of absorption by interstellar hydroxyl cations and water cations, along the sight-line to the bright continuum source W49N. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 972 GHz N = 1 - 0 transition of OH+ and the 1115 GHz 1(11) - 0(00) transition of ortho-H2O+. The resultant spectra show absorption by ortho-H2O+, and strong absorption by OH+, in foreground material at velocities in the range 0 to 70 km/s with respect to the local standard of rest. The inferred OH+/H2O+ abundance ratio ranges from ~ 3 to ~ 15, implying that the observed OH+ arises in clouds of small molecular fraction, in the 2 - 8% 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 21 cm absorption measurements, and dissimilar from that typical of other molecular tracers. The observed OH+/H abundance ratio of a few E-8 suggests a cosmic ray ionization rate for atomic hydrogen of (0.6 - 2.4) E-16 s-1, in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H3+ and other species.

arXiv commentAccepted for publication in A&A Letters

References in corpus (13)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• A model for atomic and molecular interstellar gas: The Meudon PDR code
• Black hole accretion and star formation as drivers of gas excitation and chemistry in Mrk231
• H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
• Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4
• Molecular absorption lines toward star-forming regions : a comparative study of HCO+, HNC, HCN, and CN
• Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: a ubiquitous tracer of molecular gas
• First interstellar detection of OH+
• The penetration of FUV radiation into molecular clouds
• Detection of interstellar oxidaniumyl: abundant H2O+ towards the star-forming regions DR21, Sgr B2, and NGC6334
• VLA HI Zeeman Observations Toward the W49 Complex
• Water Absorption From Line-of-Sight Clouds Toward W49A
• Time-dependent H2 formation and protonation

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