• DOI:10.1051/0004-6361/201015077
• Corpus ID: 55762914

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

@article{Neufeld2010HerschelHIFIOO,
  title={Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction},
  author={David A. Neufeld and Javier R. Goicoechea and Paule Sonnentrucker and John H. Black and Justin Pearson and S. Yu and Thomas G. Phillips and Dariusz C. Lis and Massimo De Luca and Eric Herbst and Paul B. Rimmer and Maryvonne Gerin and Tom A. Bell and François Boulanger and Jos{\'e} Cernicharo and Audrey Coutens and Emmanuel Dartois and Maja Kaźmierczak and Pierre J. Encrenaz and Edith Falgarone and Thomas R. Geballe and Thomas F. Giesen and Benjamin Godard and Paul. F. Goldsmith and C{\'e}cile Gry and H. Gupta and Patrick Hennebelle and Pierre Hily-Blant and Christine Joblin and Robert Kołos and J. Krełowski and Jes{\'u}s Mart{\'i}n-Pintado and Karl M. Menten and Raquel Rodriguez Monje and Bhaswati Mookerjea and Michel P{\'e}rault and Carina M. Persson and Rene Plume and Morvan C. Salez and Stephan Schlemmer and M. R. Schmidt and J{\"u}rgen Stutzki and David Teyssier and Charlotte Vastel and Alain Cros and K Klein and A. Lorenzani and Sabine D. Philipp and Lorene A. Samoska and Russell F. Shipman and A. Tielens and Ryszard Szczerba and Jonas Zmuidzinas},
  journal={Astronomy and Astrophysics},
  year={2010},
  volume={521}
}

• D. Neufeld, J. Goicoechea, +50 authors J. Zmuidzinas
• Published 6 July 2010
• Physics
• Astronomy and Astrophysics

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(-1) with respect to the local standard of rest. The inferred OH+/H2O+ abundance ratio ranges from similar to 3 to similar 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 x10(-8) suggests a cosmic ray ionization rate for atomic hydrogen of 0.6-2.4 x 10(-16) s(-1), in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H-3(+) and other species.