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ERIC HERBST DEPARTMENTS OF PHYSICS AND ASTRONOMY THE OHIO STATE UNIVERSITY Interstellar and Circumstellar Chemistries: The Role of Neutral-Neutral Reactions
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Dust particles contain 1% of interstellar matter.
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Cosmic rays produce ions
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Radical-Neutral Reactions Radicals: C, CN, CCH 1) Inverse T dependence 2) Large rate coefficients by 10-50 K: k 10(-10) cm 3 s -1
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(diffusion)
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FORMATION OF GASEOUS WATER H 2 + COSMIC RAYS H 2 + + e Elemental abundances: C,O,N = 10(-4); C<O H 2 + + H 2 H 3 + + H H 3 + + O OH + + H 2 OH n + + H 2 OH n+1 + + H H 3 O + + e H 2 O + H; OH + 2H, etc
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FORMATION OF O 2,N 2 CO OH + O O 2 + H OH + N NO + H NO + N N 2 + O CO, N 2 + He + C +, N + +… Precursor to ammonia, hydrocarbons CH + O CO + H
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NEUTRAL-NEUTRAL RX (CONT) CN + C 2 H 2 HCCCN + H CCH + HCN HCCCN + H YES NO CCH + C 2 H 2 C 4 H 2 + H YES O + CCH CO + CH MAYBE (E a = 250K?) k 1.2 10(-11) cm 3 s -1
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Successes for quiescent cores: (1)Reproduces 80% of abundances including ions, radicals, isomers (2)Predicts strong deuterium fractionation
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CURRENT APPROACH TO NEUTRAL-NEUTRAL RX DO NOT EXTRAPOLATE NEW LOW TEMPERATURE RESULTS UNLESS IN FAMILIES (e.g. C + HCCCN NO) 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 + C n, C n H) INCLUDES LOW RATE CONSTANT. MAINTAIN PAST ESTIMATES UNLESS CONSENSUS THAT THEY ARE INCORRECT. (E.G. O + C n, C n H) INCLUDES LOW RATE CONSTANT. ESTIMATES IMPROVED WITH IAN; WORK IN PROGRESS ESTIMATES IMPROVED WITH IAN; WORK IN PROGRESS
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IRC10216: An AGB (Old) Star Molecules and dust here C>O N,T similar to cloud UV radiation + cosmic rays LTE CO, C 2 H 2, HCN
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Actual Distributions
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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 C 2 H 2 + h CCH + H
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GROWTH OF MOLECULES. II CCH + C 2n H 2 C 2n+2 H 2 + H CN + C 2n H 2 HC 2n+1 N + H C 2n H reactions with hydrocarbons, HCN(?), HNC(?) as well as cyanoacetylenes. Benzene formed via ion-molecule rx.
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CYANOPOLYYNES in IRC Radius
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CRL618: A Protoplanetary Nebula empty Dense Thin shell 250 K Detection of benzene Photons, X-rays from central star! 100 x normal ionization rate
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Fractional Abundance
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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!!!!!
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SYNTHESIS OF BENZENE C 2 H 2 + + C 2 H 2 C 4 H 3 + + H C 4 H 3 + + C 2 H 2 c-C 6 H 5 + + h c-C 6 H 5 + + H 2 c-C 6 H 7 + + h c-C 6 H 7 + + e c-C 6 H 6 + H
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INITIAL ABUNDANCES w.r.t. H 2 CO CO C 2 H 2 C 2 H 2 HCN HCN CH 4 CH 4 NH 3 NH 3 6(-4) 6(-4) 5(-5) 5(-5) 8(-6) 8(-6) 2(-6) 2(-6)
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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.
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TYPES OF SURFACE REACTIONS REACTANTS: MAINLY MOBILE ATOMS AND RADICALS A + B AB association H + H H 2 H + X XH (X = O, C, N, CO, etc.) WHICH CONVERTS O OH H 2 O C CH CH 2 CH 3 CH 4 N NH NH 2 NH 3 CO HCO H 2 CO H 3 CO CH 3 OH X + Y XY ?????????? H + HX H 2 + X abstraction
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