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Chemical Evolution in Space: From Dark Clouds to Organics

Explore the models and history of astrochemistry from 1950 to date, focusing on molecular interactions in gas and on dust grains. Learn about reaction rates and chemistry involving species like H2, CH, and CH+, as well as the formation and destruction of complex molecules in dark and diffuse clouds. Understand the abundance and reactivity of oxygen and carbon-bearing molecules, and the formation of hydrocarbon chains through radiative association and dissociative recombination processes.

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Chemical Evolution in Space: From Dark Clouds to Organics

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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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