Astrochemistry University of Helsinki, December 2006 Lecture 2

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AstrochemistryUniversity of Helsinki, December
2006Lecture 2

• T J Millar, School of Mathematics and Physics
• Queens University Belfast,Belfast BT7 1NN,
  Northern Ireland

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Two-body reactions
Ion-neutral reactions Neutral-neutral
reactions Ion-electron dissociative
recombination (molecular ions) Ion-electron
radiative recombination (atomic ions) Radiative
association Three-body reactions (only if density
is very large)
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Formation of Molecules
Ion-neutral reactions Activation energy
barriers rare if exothermic Temperature
independent (or inversely dependent on
T) Neutral-neutral reactions Often have
activation energy barriers Often rate
coefficient is proportional to temperature
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Formation of Molecules
Radiative association A B ? AB h? Photon
emission more efficient as size of complex grows,
therefore can be important in synthesising large
molecular ions CH3 H2 ? CH5 h ? k(T) 1.3
10-13(T/300)-1 cm3 s-1 CH3 HCN ? CH3CNH h
? k(T) 9.0 10-9(T/300)-0.5 cm3 s-1
Ion-electron dissociative recombination
reactions Fast, multiple products, inverse T
dependence Atomic ion-electron radiative
recombination recombination Neutral complex
stabilises by emission of a photon, about 1000
times slower than DR rate coefficients
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Formation of H2
Gas phase association of H atoms far too slow, k
10-30 cm3 s-1
Gas and dust well-mixed In low-density gas, H
atoms chemisorb and fill all binding sites
(106) per grain Subsequently, H atoms
physisorb Surface mobility of these H atoms is
large, even at 10 K. H atoms scans surface
until it finds another atom with which it
combines to form H2
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Dense Clouds

• H2 forms on dust grains
• Ion-neutral chemistry important
• Time-scales for reaction for molecular ion M
• 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 gt
  10-13 cm3 s-1
• Reactions with X are only important if the ion
  does not react, or reacts very slowly, with H2.

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Formation of Organics

• Starts with proton transfer from H3
• C H3 ? CH H2
• CH H2 ? CH2 H
• CH2 H2 ? CH3 H
• CH3 H2 ? CH5 h?
• CH5 CO ? CH4 HCO
• C CH4 ? C2H2 H2
• C CH4 ? C2H3 H

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Formation of Hydrocarbon Chains

• C insertion
• C CmHn ? Cm1Hn-1 H
• C CmHn ? Cm1Hn-1 H
• C CmHn ? Cm1Hn-1 H
• Binary reactions
• C2H C2H2 ? C4H2 H
• C2H C2H2 ? C4H2 H
• CN C2H2 ? HC3N H

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Formation of Organics

• Radiative association
• CH3 H2O ? CH3OH2 h?
• CH3 HCN ? CH3CNH h?
• CH3 CH3OH ? CH3OCH4 h?
• Dissociative recombination
• C2H3 e- ? C2H2 H
• CH3OH2 e- ? CH3OH H
• CH3OCH4 e- ? CH3OCH3 H

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Time dependent evolution
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Water in Cold Clouds

• SWAS
• o-H2O at 557 GHz in B68 and ? Oph D

Bergin Snell, ApJ, 581, L105 (2002) Non-detecti
on of water with fractional abundances relative
to H2 of 3 10-8 (B68) and 6 10-9 (? Oph D)
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Oxygen Chemistry

• H3 O ? OH H2 M
• 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, .. ,

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Oxygen Chemistry

• O OH ? H O2 M for T gt 160K, fast
• C OH ? H CO
• N OH ? H NO M for T gt 100K, fast
• S OH ? H SO M at T 300K, fast
• Si OH ? H SiO
• C O2 ? CO O M for T gt 15K, fast

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Oxygen Chemistry

• Conclude
• We should be able to explain the abundances of
  H2O (all reactions measured)
• - of OH (no i-n reactions measured, important
  n-n reactions measured)
• - of O2 (all reactions measured)
• But we cannot !!!

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Results
Oxygen chemistry O2 abundance to 10-4 - 100
times larger than observed H2O abundance close
to 10-6 - 100 times larger than observed
T 10K, n(H2) 104 cm-3
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Water in shocks

• SWAS observations of IC443

Snell et al. ApJ, 620, 758 (2005) o-H2O/CO 2
10-4 3 10-3 Or o-H2O/H2 10-8 Again, seemingly
a big discrepancy between observation ands
theory Fast J shocks too little H2 IR, ok for
H2O Slow J shocks cannot produce H2 and OI
emission, too much water Fast C shock cannot
produce H2 and OI emission, too much water Slow C
shock too little H2 IR, ok for H2O, too little
CII
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Water in shocks

• SWAS observations of IC443

Fundamental problem H2 IR emission requires T
1000 K At these temperatures all O not in CO is
converted to H2O Solutions(?) (1) Large H
abundance doesnt work (2) Freeeze H2O when
gas cools doesnt work (3) Freeze all free O
as H2O before the shock arrives (4)
Photodissociative H2O with UV photons produced in
fast shock (5) Shocks are not in
steady-state (6) Several types of shock are
present
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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
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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 gtgt 1 when kT ltlt ?E0
kf kL kr ltlt kf
(Gerlich, Roueff)
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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
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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
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Fractionation in D2CO
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The UMIST Database for Astrochemistry 2005
www.udfa.net
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Summary
4566 binary reactions among 420 species 13
elements H, He, C, N. O, F, Na, Mg, Si, P, S,
Cl, Fe Fluorine included for first time 544
neutral-neutral reactions - 269 measured 2939
ion-neutral reactions - 1157 measured 485
dissociative recombination reactions/product
channels 95 measured 25 radiative
recombinations 90 radiative associations 11
cosmic-ray ionisation reactions 156
cosmic-ray-induced photoreactions 216
photoreactions 32 ternary reactions Database
including ion-dipole enhanced rate coefficients
also available
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Summary