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ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY

Chemistry in Interstellar Space. ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY. MOLECULAR ROTATION. “radio” emissions. D E = h n. MOLECULAR VIBRATIONS. Infrared absorption. Cosmic rays produce ions. Radical-Neutral Reactions.

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ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY

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  1. Chemistry in Interstellar Space ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY

  2. MOLECULAR ROTATION “radio” emissions DE = hn

  3. MOLECULAR VIBRATIONS Infrared absorption

  4. Cosmic rays produce ions

  5. Radical-Neutral Reactions Radicals: C, CN, CCH 1) Inverse T dependence 2) Large rate coefficients by 10-50 K: k~ 10(-10) cm3 s-1

  6. FORMATION OF GASEOUS WATER H2 + COSMIC RAYS  H2+ + e Elemental abundances: C,O,N = 10(-4); C<O Elemental abundances: C,O,N = 10(-4); C<O H2+ + H2 H3+ + H H3+ + O  OH+ + H2 OHn+ + H2  OHn+1+ + H H3O+ + e  H2O + H; OH + 2H, etc

  7. FORMATION OF HYDROCARBONS H3+ + C  CH+ + H2 CHn+ + H2  CHn+1+ + H; n=1,2 CH3+ + H2  CH5+ + hn CH5+ + e  CH4 + H (5%)  CH3 + 2H (70%) CH5+ + CO  CH4 + HCO+

  8. FORMATION OF O2 ,N2 CO OH + O  O2 + H OH + N  NO + H NO + N  N2 + O CH + O  CO + H CO, N2 + He+ C+, N+ +… Precursor to ammonia, hydrocarbons

  9. NEUTRAL-NEUTRAL RX (CONT) CN + C2H2 HCCCN + H YES CCH + C2H2 C4H2 + H YES CCH + HCN  HCCCN + H NO O + CCH  CO + CH k = 1.2 10(-11) cm3 s-1 MAYBE (Ea = 250K?)

  10. Latest network – osu.2003 – contains over 300 rapid neutral-neutral reactions. Rate coefficients estimated by Ian Smith and others.

  11. (diffusion)

  12. TYPES OF SURFACE REACTIONS REACTANTS: MAINLY MOBILE ATOMS AND RADICALS A + B AB association H + H H2 H + X XH (X = O, C, N, CO, etc.) WHICH CONVERTS O  OH  H2O C CH  CH2  CH3  CH4 N  NH  NH2  NH3 CO  HCO  H2CO  H3CO  CH3OH X + Y XY ??????????

  13. MODELLING DIFFUSIVE SURFACE CHEMISTRY Rate Equations - kcrdNH Only accurate if there are lots of reactive species on every dust particle.

  14. GRAIN MANTLE GROWTH (COLD CLOUDS; silicate grains)

  15. % Agreement in TMC-1 Gas-phase species Roberts & Herbst 2002

  16. Other Approaches • Monte Carlo method • Modified rate method (semi-empirical) • Probabilistic master equation Second method changes rate coefficients so that fractional abundances do not exist. Last method follows probabilities for specific numbers of species; easily coupled with rate equations for the gas phase but computationally intensive.

  17. PROBABILISTIC MASTER EQUATION

  18. Some Outstanding Astrochemical Problems • How to make gas-phase models more robust • How to construct gas-grain models and predict mantle abundances accurately • How to model the chemistry of star- and planet-forming regions (heterogeneity and time dependence)

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