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ISM & Astrochemistry Lecture 3

ISM & Astrochemistry Lecture 3. Models - History. 1950-1972 – Grain surface chemistry – H 2 , CH, CH + 1973-1990 – Ion-neutral chemistry – HD, DCO + 1990-2000 – Neutral-neutral chemistry – HC 3 N 2000-date – Gas/Grain interaction – D 2 CO, ND 3 10,000 reactions, 500 species.

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ISM & Astrochemistry Lecture 3

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  1. ISM & AstrochemistryLecture 3

  2. Models - History 1950-1972 – Grain surface chemistry – H2, CH, CH+ 1973-1990 – Ion-neutral chemistry – HD, DCO+ 1990-2000 – Neutral-neutral chemistry – HC3N 2000-date – Gas/Grain interaction – D2CO, ND3 10,000 reactions, 500 species

  3. Dark Clouds • H2 forms on dust grains • Ion-neutral chemistry important • Time-scales for reaction for molecular ion M+ - 1/kn(X) • 109/n(H2) for fast reaction with H2 • 106/n(e) for fast dissociative recombination with electrons • 109/n(X) for fast reaction with X Since n(e) ~ 10-8n, dissociative recombination is unimportant for ions which react with H2 with k > 10-13 cm3 s-1; Reactions with X are only important if the ion does not react, or reacts very slowly, with H2 since n(X) = 10-4n(H2) at most.

  4. Fractional Ionisation H2 + crp  H2+ + e k1 - cosmic ray ionisation H2+ + H2  H3+ + H k2 H3+ + X  XH+ + H2 k3 XH+ + e  neutral products k4 - dissociative recombination Consider XH+: Steady-state: formation rate = destruction rate k1n(H2) = k4n(XH+)n(e) Zero-order approximation: Assume n(XH+) = n(e)

  5. Fractional Ionisation Then, the fractional ionisation, f(e), can be written: f(e) = n(e)/n(H2) = [k1/k4n(H2)]1/2 Put in rate coefficients: k1 = 10-17 s-1, k4 = 10-7 cm3 s-1 Then f(e) = 10-5/n1/2(H2) i.e. f(e) ~ 10-7 – 10-8 for n(H2) ~ 104-105 cm-3 in dark clouds

  6. Oxygen Chemistry H3+ + O  OH+ + H2 M - measured OH+ + H2  H2O+ + H M H2O+ + H2  H3O+ + H M H3O+ + e  O, OH, H2O M Destruction of H2O: He+, C+, H3+, HCO+, .. (M) Destruction of OH: He+, C+, H3+, HCO+, .. ,

  7. Oxygen Chemistry OH is a very reactive radical O + OH  H + O2 M for T > 160K, fast C + OH  H + CO N + OH  H + NO M for T > 100K, fast S + OH  H + SO M at T = 300K, fast Si + OH  H + SiO C + O2  CO + O M for T > 15K, fast CO is the most abundant IS molecule – after H2 n(CO) ~ 10-5-10-4 n(H2)

  8. Results Oxygen chemistry O2 abundance to 10-4 - ~ 100 times larger than observed H2O abundance close to 10-6 - ~ 100 times larger than observed PROBLEM!! T = 10K, n(H2) = 104 cm-3

  9. Carbon Chemistry (diffuse clouds) C+ + H2  CH+ + H endoergic by about 0.4eV (4640K) C+ + H2  CH2+ + hnu theory – k~ 10-16 cm3 s-1 CH2+ + H2  CH3+ + H M – k ~ 10-9 cm3 s-1 CH3+ + e  products M – k1 ~ 10-7 cm3 s-1 CH3+ + hnu  products M – k2 ~ 10-9 s-1 (unshielded) CH3+ + H2  CH5+ + hnu M – k3 ~ 10-13 cm3 s-1 Loss of CH3+: k1n(e) vs k2 vs vs k3n(H2) n(e) = n(C+) = 10-4n; n(H2) = 0.01n (typically); n ~ 100 cm-3 Loss of CH3+: 10-9 vs 10-9 vs 10-13 (s-1), So reactions 1 & 2 dominate, DR and UV win and prevents complex molecule formation – Molecules in diffuse clouds are relatively simple (few atoms)

  10. Carbon Chemistry (dark clouds) H3+ + C  CH+ + H2 M - measured CH+ + H2  CH2+ + H M CH2+ + H2  CH3+ + H M CH3+ + H2  CH4+ + H Endoergic, but … CH3+ + H2  CH5+ + hnu M – slow (4 10-13 cm3 s-1) CH5+ + e  CH, CH2, CH3 (mostly), CH4 M CH5+ + CO  CH4 + HCO+ M – dominant loss for CH5+ Destruction of CH4: He+, C+, H3+, HCO+, .. (M)

  11. Abundance of Methane H3+ + C  … CH4 k1 = 10-9 cm3 s-1 CH4 + X+  products k2 = 10-9 cm3 s-1 Destruction of CH4: He+, C+, H3+, HCO+, .. (M) Steady-state: Formation rate = destruction rate k1n(C)n(H3+) = k2n(X+)n(CH4) n(CH4)/n(C) = n(H3+)/n(X+) ~ 0.1 A significant fraction of C atoms is converted to methane

  12. Formation of Organics Starts with proton transfer from H3+ C + H3+ CH+ + H2 CH+ + H2 CH2+ + H CH2+ + H2 CH3+ + H CH3+ + H2 CH5+ + hυ CH5+ + CO  CH4 + HCO+ C+ + CH4 C2H2+ + H2 C+ + CH4 C2H3+ + H

  13. Formation of Hydrocarbon Chains C insertion: C + CmHn+ Cm+1Hn-1+ + H C+ + CmHn Cm+1Hn-1+ + H C + CmHn Cm+1Hn-1 + H Binary reactions: C2H + C2H2+ C4H2+ + H C2H + C2H2 C4H2 + H CN + C2H2 HC3N + H Carbon and carbon-bearing molecules are very reactive with each other They are not reactive with H2, most reactions are endoergic So carbon-chains build easily in cold, dark clouds – as observed

  14. Formation of Organics Radiative association: CH3+ + H2O  CH3OH2+ + hnu CH3+ + HCN  CH3CNH+ + hnu CH3+ + CH3OH  CH3OCH4+ + hnu Dissociative recombination: C2H3+ + e- C2H2 + H CH3OH2+ + e- CH3OH + H CH3OCH4+ + e- CH3OCH3 + H RA reactions occur faster for larger systems – in many cases each collision leads to a product – compare with C+ and H2, where only 1 in 107 collisions produces CH2+ In DR many product channels can occur, the ‘preferred’ channel might actually be a minor channel.

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