Other
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280e..71n&link_type=abstract
The Molecular Universe, Proceedings of the 280th Symposium of the International Astronomical Union held in Toledo, Spain, May 30
Other
Scientific paper
Recent Herschel observations, performed with the HIFI instrument as part of the PRISMAS key program, have led to the detection of the OH+ and H2O+ molecular ions in absorption toward the bright continuum sources W31C (G10.6-0.4), W49, W51, and DR21(OH). The observed absorption, typically observed over a range of Doppler velocities, arises primarily in foreground diffuse material unassociated with these continuum sources, and permits the OH+ and H2O+ column densities to be determined in multiple velocity components that typically intersect the sight-line to any source. The observed OH+ and H2O+ column densities can be interpreted with the use of detailed chemical models for diffuse molecular clouds, to be discussed in this presentation, along with simple analytic expressions relating the molecular abundances to the physical and chemical conditions in the absorbing region. Typical OH+ / H2O++ column density ratios ˜ 6 imply that the observed OH+ arises in clouds that are primarily ATOMIC, with a molecular fraction of just a few percent. 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. Typical OH+/H abundance ratios of a few × 10-8 suggest a cosmic ray ionization rate for atomic hydrogen ˜ 10-16 s-1, in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H3+ and other species.
Gerin Maryvonne
Goicoechea Javier R.
Hollenbach David
Kaufman Maike
Neufeld David
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