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Nuclear Spin of H 3 + and H 2 in Dense Molecular Clouds

Nuclear Spin of H 3 + and H 2 in Dense Molecular Clouds. Kyle N. Crabtree and Benjamin J. McCall. Dense clouds and deuterium fractionation. Cold (10-50 K), dense ( ~ 10 4 cm -3 ) clouds of molecular gas and dust

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Nuclear Spin of H 3 + and H 2 in Dense Molecular Clouds

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  1. Nuclear Spin of H3+ and H2 in Dense Molecular Clouds Kyle N. Crabtree and Benjamin J. McCall

  2. Dense clouds and deuterium fractionation • Cold (10-50 K), dense (~104 cm-3) clouds of molecular gas and dust • Cosmic D/H ratio ~10-5, but some molecules have larger observed ratios (e.g. DCO+/HCO+~0.2 in L134N) • What causes this enrichment? L183 (L134N) NASA/JPL, Spitzer, L. Pagani Barnard 68 ESO, VLT, FORS team http://bjm.scs.illinois.edu

  3. Chemistry of deuterium fractionation Predominant form of D in molecular clouds is HD “Extracting” D from HD: H3+ + HD  H2D+ + H2 + 232 K H2D+ + X (DX+ + H2) + (HX+ + HD) Further enrichment: H2D+ + HD  D2H+ + H2 + 187 K D2H+ + HD  D3+ + H2 + 234 K http://bjm.scs.illinois.edu

  4. o-H3+ (J,K) = (1,0) o-H2; J = 1 DE 32 K DE 170 K p-H3+ (J,K) = (1,1) p-H2; J = 0 Nuclear spin modifications of H3+ and H2 Chemical reactions interconvert o- and p-H2 (o- and p-H3+) e.g. o-H2 + H+ p-H2 + H+; o-H3+ + p-H2  p-H3+ + o-H2 Non-equilibrium distributions very possible http://bjm.scs.illinois.edu

  5. o-H2 influences deuterium fractionation Deuterium fractionation reactions: H3+ + HD  H2D+ + H2 + 230 K H2D+ + HD  D2H+ + H2 + 190 K D2H++ HD  D3+ + H2 + 230 K But, with o-H2, reverse reactions become more feasible: H2D+ + o-H2 + 60 K  H3+ + HD D2H+ + o-H2 + 20 K  H2D+ + HD D3+ + o-H2 + 60 K  D2H+ + HD Also, nuclear spin modifications of H3+, H2D+, etc. are important Abundance of o-H2 is critical for deuterium fractionation http://bjm.scs.illinois.edu

  6. H3+/H2 chemistry in dense clouds H + H  H2 (grain surface formation) H2 + H+  H+ + H2 (H2 o/p conversion) H2  H2+ + e- (cosmic ray ionization/H2 destruction) H2+ + H2  H3+ + H (H2 destruction/H3+ formation) H3+ + H2  H2 + H3+ (H3+ and H2 o/p conversion) H3+ + CO  HCO+ + e- (H3+ destruction) H3+ + HD  H2D+ + H2 (start of D fractionation) CO/HD ratio impacts extent of D fractionation http://bjm.scs.illinois.edu

  7. Dense clouds vs. cores H3+ + CO  HCO+ + H2 H3+ + HD  H2D+ + H2 T < 10 K CO frozen! T = 10-50 K CO/HD > 10 CO/HD < 1 H3+ + HD  H2D+ + H2 H3+ + CO  HCO+ + H2 http://bjm.scs.illinois.edu

  8. Nuclear spin dependent chemistry in dense clouds H 75% 25% H+ o-H2 p-H2 X 50/50 H3+ o-H2+ H2+ X H2+ o-H3+ p-H3+ H2 http://bjm.scs.illinois.edu

  9. Database of reaction rates for o/p H3+ and H2 • Inspired by OSU and UMIST databases • 28 species, 172 reactions, 220 rates • New platform-independent C++/Qtsolving code w/ GNU Scientific Library differential equation solving algorithms http://bjm.scs.illinois.edu

  10. Sample model output: abundances of major species H2 H O COO2 C e- H3+ H+ http://bjm.scs.illinois.edu

  11. Timescale for o/p-H2 steady state: 107yr 0.9999997 @ 10 K p2 = 0.995 0.92 @ 10 K p3 = 0.68 T(H3+) = 24 K T(H2) = 21 K http://bjm.scs.illinois.edu

  12. Trends observed in modeling http://bjm.scs.illinois.edu The o-H2 fraction is always greater than equilibrium value The timescale for o/p-H2 steady state is (nearly) independent of density Higher temperature and cosmic ray ionization rate decrease timescale The lower the temperature, the more nonthermal T(H2) is at steady state The proton abundance may play a key role (oxygen chemistry)

  13. Proton abundance and oxygen chemistry O2 H2O H3+ H+ H+ + O2 O2+ + H H+ + H2O  H2O+ + H Modeling nuclear spin dependent chemistry is critical, and much work still needs to be done http://bjm.scs.illinois.edu

  14. Acknowledgements NASA Earth and Space Science Fellowship Program http://bjm.scs.illinois.edu

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