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Interstellar and Circumstellar Chemistries: The Role of Neutral-Neutral Reactions

Interstellar and Circumstellar Chemistries: The Role of Neutral-Neutral Reactions. ERIC HERBST DEPARTMENTS OF PHYSICS AND ASTRONOMY THE OHIO STATE UNIVERSITY. Dust particles contain 1% of interstellar matter. Cosmic rays produce ions. Radical-Neutral Reactions. Radicals: C, CN, CCH.

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Interstellar and Circumstellar Chemistries: The Role of Neutral-Neutral Reactions

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  1. Interstellar and Circumstellar Chemistries: The Role of Neutral-Neutral Reactions ERIC HERBST DEPARTMENTS OF PHYSICS AND ASTRONOMY THE OHIO STATE UNIVERSITY

  2. Dust particles contain 1% of interstellar matter.

  3. Cosmic rays produce ions

  4. 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

  5. (diffusion)

  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 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

  8. 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?)

  9. Successes for quiescent cores: • Reproduces 80% of abundances including ions, radicals, isomers • Predicts strong deuterium fractionation

  10. CURRENT APPROACH TO NEUTRAL-NEUTRAL RX • DO NOT EXTRAPOLATE NEW LOW TEMPERATURE RESULTS UNLESS IN FAMILIES (e.g. C + HCCCN NO) • MAINTAIN PAST ESTIMATES UNLESS CONSENSUS THAT THEY ARE INCORRECT. (E.G. O + Cn, CnH) INCLUDES LOW RATE CONSTANT. • ESTIMATES IMPROVED WITH IAN; WORK IN PROGRESS

  11. IRC10216: An AGB (Old) Star Molecules and dust here LTE C>O N,T similar to cloud CO, C2H2, HCN UV radiation + cosmic rays

  12. Actual Distributions

  13. GROWTH OF MOLECULES Occurs via neutral and ionic (+ and -) reactions. Modified network necessary to account for acetylenic chemistry. Photochemistry important in the production of radicals such as CN and CCH C2H2 + hn CCH + H

  14. GROWTH OF MOLECULES. II CCH + C2nH2 C2n+2H2 + H CN + C2nH2 HC2n+1N + H C2nH reactions with hydrocarbons, HCN(?), HNC(?) as well as cyanoacetylenes. Benzene formed via ion-molecule rx.

  15. CYANOPOLYYNES in IRC Radius 

  16. CRL618: A Protoplanetary Nebula Detection of benzene empty 250 K Dense Thin shell Photons, X-rays from central star! 100 x normal ionization rate

  17. Fractional Abundance 

  18. SUMMARY Thanks to Ian Smith and others (Bertrand Rowe, Ian Sims, David Clary etc.), we now know that neutral-neutral reactions are competitive in both oxygen-rich and carbon-rich chemistries in interstellar and circumstellar sources!!!!!

  19. SYNTHESIS OF BENZENE C2H2+ + C2H2 C4H3+ + H C4H3+ + C2H2 c-C6H5+ + hn c-C6H5+ + H2 c-C6H7+ + hn c-C6H7+ + e  c-C6H6 + H

  20. CO C2H2 HCN CH4 NH3 6(-4) 5(-5) 8(-6) 2(-6) 2(-6) INITIAL ABUNDANCES w.r.t. H2

  21. ACTUAL CLOUD CORES Model TMC-1 L134N NSM/+grains 80% 80% NNM 50% 70% NOTES: (1) C=0.42 x O (2) N-N reactions involving C and O most important. (3) Larger molecules more strongly affected.

  22. 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 ?????????? H + HX H2 + X abstraction

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