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

Nature volume 422, pages 500–502 (2003)Cite this article

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.

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Figure 1: Spectra of two H3+ transitions arising from the two lowest rotational levels, which are the only levels with significant population in diffuse clouds. R(1,1)u originates from the lowest para level (J = 1, K = 1), while R(1,0) comes from the lowest ortho level (J = 1, K = 0).
Figure 2: Measured dissociative recombination rate coefficient of rotationally cold H3+ as a function of detuning energy.
Figure 3: Calculated thermal rate coefficient ke for the dissociative recombination of rotationally cold H3+ ions as a function of electron temperature, based on CRYRING measurements.

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Acknowledgements

B.J.M. and A.J.H. thank K. Wilson and C.-Y. Chung for their assistance in the laboratory, D. Lucas for the loan of equipment, and H. Chan and E. Granlund for their support. B.J.M. thanks T. Oka, L. M. Hobbs, D. G. York and T. P. Snow for conversations about the ζ Persei line of sight. We thank C. H. Greene for providing us with the results of his calculations in advance of publication. M.L. thanks D. Zajfman for information on TSR results before publication. The idea of using a supersonic expansion source for dissociative recombination measurements originated during conversations between B.J.M. and C. M. Lindsay, and the experiment was designed by B.J.M., A.J.H. and M.L. The supersonic expansion ion source was built and spectroscopically characterized by A.J.H. and B.J.M. in the laboratory of R.J.S. in Berkeley. The dissociative recombination measurements in Stockholm were carried out by all of the authors except R.J.S. and T.R.G. The UKIRT observations were obtained by B.J.M. and T.R.G. The authors thank the staff of the Manne Siegbahn Laboratory for help with the experiment. B.J.M. is supported by the Miller Institute for Basic Research in Science. A.J.H. and R.J.S. acknowledge support from the AFOSR and NSF. T.R.G. is supported by Gemini Observatory, operated by AURA for the international Gemini partnership. N.D. and G.H. were supported in part by the US DOE, Office of Fusion Energy. J.S. is supported in part by the State Committee for Scientific Research. Support is acknowledged from the EU fifth framework program, the EOARD, the Swedish Research Council and STINT.

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Authors and Affiliations

  1. Department of Astronomy, University of California at Berkeley, Berkeley, California, 94720, USA

    B. J. McCall

  2. Gemini Observatory, 670 North A'ohoku Place, Hilo, Hawaii, 96720, USA

    T. R. Geballe

  3. JILA, University of Colorado and National Institute of Standards and Technology, Boulder, Colorado, 80309, USA

    N. Djuric & G. H. Dunn

  4. Institute of Physics, Świetokrzyska Academy, 25 406, Kielce, Poland

    J. Semaniak & O. Novotny

  5. Department of Electronics and Vacuum Physics, Faculty of Mathematics and Physics, Charles University Prague V Holesovickach 2, 8, Prague, Czech Republic

    O. Novotny

  6. Department of Physics, SCFAB, Stockholm University, S-106 91, Stockholm, Sweden

    A. Al-Khalili, A. Ehlerding, F. Hellberg, S. Kalhori, A. Neau, R. Thomas & M. Larsson

  7. Manne Siegbahn Laboratory, Stockholm University, S-104 05, Stockholm, Sweden

    F. Österdahl

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  1. B. J. McCall
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Correspondence to B. J. McCall.

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McCall, B., Huneycutt, A., Saykally, R. et al. An enhanced cosmic-ray flux towards ζ Persei inferred from a laboratory study of the H3+–e- recombination rate. Nature 422, 500–502 (2003). https://doi.org/10.1038/nature01498

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