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ENHANCED COSMIC-RAY FLUX TOWARD $\zeta$ PERSEI INFERRED FROM STORAGE RING MEASUREMENT OF DISSOCIATIVE RECOMBINATION RATE OF ROTATIONALLY COLD $H^{+}_{3}$

Please use this identifier to cite or link to this item: http://hdl.handle.net/1811/20735

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Title: ENHANCED COSMIC-RAY FLUX TOWARD $\zeta$ PERSEI INFERRED FROM STORAGE RING MEASUREMENT OF DISSOCIATIVE RECOMBINATION RATE OF ROTATIONALLY COLD $H^{+}_{3}$
Creators: McCall, Benjamin J.; Huneycutt, A. J.; Saykally, R. J.; Geballe, T. R.; Djuric, N.; Dunn, G. H.; Semaniak, J.; Novotny, O.; Al-Khalili, A.; Ehlerding, A.; Hellberg, F.; Kalhori, S.; Neau, A.; Thomas, R.; Osterdahl, F.; Larsson, M.
Issue Date: 2003
Abstract: The $H^{+}_{3}$ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many interstellar molecules. In dense clouds, the $H_{3}^{+}$ abundance is understood using a simple chemical model, from which observations of $H_{3}^{+}$ yield valuable estimates of cloud path length, density, and temperature. On the other hand, observations of diffuse clouds have suggested that $H_{3}^{+}$ is considerably more abundant than expected from the chemical models. However, diffuse cloud models have been hampered by the uncertain values of three key parameters: the rate of $H_{3}^{+}$ destruction by electrons, the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the $H_{3}^{+}$ dissociative recombination rate under nearly interstellar conditions, using a supersonic expansion discharge source that has been shown (using cavity ringdown spectroscopy) to produce rotationally cold $H_{3}^{+}$ ions. We also report the observation of $H_{3}^{+}$ in a diffuse cloud (towards $\zeta$ Persei) where the electron fraction is already known from ultraviolet spectroscopy. Taken together, these results allow us to derive the value of the third uncertain model parameter: we find that the cosmic-ray ionization rate in this sightline is forty times faster than previously assumed. If such a high cosmic-ray flux is indeed ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of $H_{3}^{+}$ can be resolved.
URI: http://hdl.handle.net/1811/20735
Other Identifiers: 2003-FA-04
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