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ISM & Astrochemistry Lecture 4

This lecture explores nitrogen and sulfur chemistry in dark clouds, highlighting the endothermic and exothermic reactions that govern molecular formation. It details the challenging creation of N-H and S-H bonds in low-temperature environments, utilizing reactive intermediates like H3+ and OH radicals. The essential processes leading to ammonia (NH3) formation from nitrogen and the role of deuterium (D) in primordial nucleosynthesis are discussed. Observations of deuterated species, such as ND3 and D2CO, reveal the dynamics of chemical transformations in interstellar environments.

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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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