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ISM & Astrochemistry Lecture 4

ISM & Astrochemistry Lecture 4. Nitrogen Chemistry (dark clouds). H 3 + + N  NH + + H 2 Endothermic by ~ 100K N + + H 2  NH + + H Endothermic So, at low temperatures N-H bonds are very difficult to make Use the reactive OH radical to make N 2 as an intermediate:

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ISM & Astrochemistry Lecture 4

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  1. ISM & AstrochemistryLecture 4

  2. Nitrogen Chemistry (dark clouds) H3+ + N  NH+ + H2 Endothermic by ~ 100K N+ + H2  NH+ + H Endothermic So, at low temperatures N-H bonds are very difficult to make Use the reactive OH radical to make N2 as an intermediate: N + OH  NO + H NO + N  N2 + O

  3. Nitrogen Chemistry (dark clouds) In dark clouds, N2 is destroyed by He+: He+ + N2  N+ + N + He Exothermic, fast The excess of this reaction, since it produces atomic particles, results in species with excess kinetic energy, much greater than the thermal energy at 10K Kinetically excited N+ can now react with H2 to form NH+. Subsequent reactions with H2 form NH4+ which undergoes DR to produce NH3. So, NH3 needs N2 to be formed first – it is a late time molecule – and its formation is inefficient. H3+ + N2  N2H+ + H2 N2H+ observed in IS clouds

  4. Sulphur Chemistry S+ + H2  SH+ + H Endothermic by ~ 0.4eV H3+ + S  SH+ + H2 Exothermic SH+ + H2  H2S+ + H Endothermic H2S+ + H2  H3S+ + H Endothermic So, S-H bonds are very difficult to make in interstellar clouds unless the appreciable exothermicities can be overcome. Again can use OH to make oxides: S + OH  SO + H And CH to make carbides: S + CH  CS + H

  5. Deuterium and the Big Bang For temperatures larger than 109 K, a neutron and a proton can fuse to produce a deuteron p + n  2H(D) The amount of Deuterium formed is sensitive to the baryon density - if large, then D fuses with another D to produce Helium and D is destroyed quickly - if small, then D does not get destroyed and He doe not form efficiently D/H ratio is a sensitive measure of the amount of mass in the Universe

  6. Nucleosynthesis in the Big Bang

  7. D/H Abundance If the Universe has enough mass the expansion we see can be reversed by gravity. At ‘critical mass’ the Universe will expand to a maximum size and stop. D/H is a probe of this density.

  8. D/H Abundance Measurements Earth’s Oceans – 1.5 10-4 Neptune, Uranus, Titan – 1-2 10-4 Comet Halley – 4 10-4 Jupiter, Saturn – 2 10-5 Nearby Interstellar Medium – 1-3 10-5

  9. ND3 in Interstellar Clouds Submillimetre detection of ND3 by Lis et al., Astrophysical Journal, 571, L55 (2002) ND3/NH3 = 8 10-4, compared with (D/H)3 ~ 3 10-15

  10. Deuterium in Interstellar Clouds

  11. Thermodynamic Effect Consider, D/H exchange reaction: kf A-H+ + B-D A-D+ + B-H + E0 kr trace reservoir trace reservoir HD, D K(T) = kf /kr >> 1 when kT << E0 kf = kL; kr << kf (Gerlich, Roueff)

  12. Important Fractionation Reactions Criteria: neutral abundant, ion reasonably abundant; forward rate coefficient large (i) H3+ + HD H2D+ + H2 + 220 K (ii) CH3+ + HD CH2D+ + H2 + 375 K (iii) C2H2+ + HD C2HD+ + H2 + 550 K (iv) H3+ + D H2D+ + H + 632 K (v) OH + D OD + H + 810 K

  13. Enhancement Factors Consider reaction (i): (i) H3+ + HD H2D+ + H2 + 220 K At steady state: k1fn(H3+)n(HD) = k1rn(H2D+)n(H2) = k1fexp(-220/T)n(H2D+)n(H2) n(H3D+)/n(H3+) = [n(HD)/n(H2)] exp(220/T) = 2(D/H)cosmicexp(220/T) = S1(T)2(D/H)cosmic So, potential for D to be enhanced exponentially in molecules at low T (<< 220K)

  14. Secondary Fractionation Once formed H2D+ can transfer its deuteron to other species H2D+ + CO(N2, ..)  DCO+(N2D+, ..) + H2 H2D+ + CO(N2, ..)  HCO+(N2H+, ..) + HD Statistically, DCO+(N2D+, ..) formed in 1/3 of reactions, leads to the enhancement of D in DCO+ (N2D+, ..) at one-third the level of that in H2D+. Important because these molecular ions are easy to observe

  15. Enhancement Factors Since H2D+ can react with other species – most importantly electrons and other neutral molecules, CO, N2, H2O, … So, we can write the enhancement factor more completely as (M = neutral molecule) At low temperatures, k1r tends to zero and we can use this to derive a constraint on f(e), the fractional abundance of electrons

  16. Electron Fraction f(e) ~ 10-7 – 10-8 in dark clouds

  17. Enhancement Factors – Depleted Cores

  18. D2CO and N2D+ in IRAS 16293

  19. Multiple Deuteration • H2D+ + HD D2H+ + H2 + 187 K • H2D+ + HD D3+ + H2 + 234 K • CH2D+ + HD CHD2+ + H2 + 370 K • CHD2+ + HD CD3+ + H2 + 370 K Model: 340 species – 125 singly deuterated, 30 doubly deuterated, 17 triply deuterated, 5 with 4 or more D atoms ~ 10,500 reactions linking these species

  20. High density, low T drives multiple deuteration HD,D2,D,H D2,D H2,H HD,H2 D,H e- H2 HD2+ D3+ H3+ H2D+ HD CO,N2,O DCO+,N2D+,OD+ DCO+,HCO+,N2D+,N2H+,OD+,OH+ HCO+,N2H+,OH+

  21. Fractionation in D2CO

  22. Results from a pseudo-time dependent model with T=10K, n(H2)=106 cm-3 Fractional abundances varying over time Molecular D/H ratios • At late times the abundance of H2D+ is similar to HD2+ : this prediction was confirmed by Vastel et al. (2004) • H2D+ ~ HD2+ ~ H3+, as seen by Caselli et al. (2003) towards the prestellar core L1544. • D3+ becomes the most abundant deuterated molecule (after HD). • The atomic D/H ratio rises to ~0.8: important for surface chemistry

  23. Observation of H2D+

  24. Observation of D2H+

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