An enhanced cosmic-ray flux towards ζ Persei inferred from a laboratory study of the H3+–e- recombination rate

B. J. McCall (), A. J. Huneycutt, R. J. Saykally, T. R. Geballe, N. Djuric, G. H. Dunn, J. Semaniak, O. Novotny, A. Al-Khalili, A. Ehlerding, F. Hellberg, S. Kalhori, A. Neau, R. Thomas, F. Österdahl and M. Larsson
Additional contact information
B. J. McCall: University of California at Berkeley
T. R. Geballe: Gemini Observatory
N. Djuric: JILA, University of Colorado and National Institute of Standards and Technology
G. H. Dunn: JILA, University of Colorado and National Institute of Standards and Technology
J. Semaniak: Institute of Physics, Świetokrzyska Academy
O. Novotny: Institute of Physics, Świetokrzyska Academy
A. Al-Khalili: Stockholm University
A. Ehlerding: Stockholm University
F. Hellberg: Stockholm University
S. Kalhori: Stockholm University
A. Neau: Stockholm University
R. Thomas: Stockholm University
F. Österdahl: Stockholm University
M. Larsson: Stockholm University

Nature, 2003, vol. 422, issue 6931, 500-502

Abstract: Abstract The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules1,2. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature3,4. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models5,6,7. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards ζ Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations5,6,7 of H3+ can be resolved.

Date: 2003
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DOI: 10.1038/nature01498

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