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The Subaru Seminar is usually held in Room 104 of the Hilo Base Facility, adjacent to the main lobby. Everyone is welcome to attend. If you are interested in giving a seminar, please contact Subaru seminar organizers (Nagayoshi Ohashi, Tae-Soo Pyo, Sherry Yeh) by email : (please change"_at" to @).

October 08, Tuesday, at 11:00 am

" OH+ and H2O+: Probes of the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate "

Nick Indriolo

(Johns Hopkins University)

The fast ion-molecule chemistry that occurs in the interstellar medium (ISM) is initiated by cosmic-ray ionization of both atomic and molecular hydrogen. Species that are near the beginning of the network of interstellar chemistry such as the oxygen-bearing ions OH+ and H2O+ can be useful probes of the cosmic-ray ionization rate. This parameter is of particular interest as, to some extent, it controls the abundances of several molecules. Using observations of OH+ and H2O+ made with HIFI on board Herschel through the PRISMAS and WISH key programs and the open time program OT1_dneufeld_1, we have inferred the cosmic-ray ionization rate of atomic hydrogen in multiple distinct clouds along 12 Galactic sight lines. These two molecules also allow us to determine the molecular hydrogen fraction (amount of hydrogen nuclei in H2 versus H) as OH+ and H2O+ (and H3O+ if observed) abundances are dependent on the competition between dissociative recombination with electrons and hydrogen abstraction reactions involving H2. Our observations of OH+ and H2O+ indicate environments where H2 accounts for less than 10% of the available hydrogen nuclei, suggesting that these species primarily reside in the diffuse, atomic ISM. Most previous estimates of the cosmic-ray ionization rate have relied on molecules that form in regions more dominated by molecular hydrogen---e.g., H3+ and HCO+---so these observations have allowed us to extend the investigation of the cosmic-ray flux into less explored environments. Average ionization rates in the diffuse, atomic ISM are on the order of a few times 10-16 s-1, with most values in specific clouds above or below this average by a factor of 3 or so. This result is in good agreement with the most up-to-date determination of the distribution of cosmic-ray ionization rates in diffuse molecular clouds as inferred from observations of H3+. Without question, Herschel has improved our overall understanding of interstellar chemistry and the ISM, especially in primarily neutral atomic clouds.

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