ISM & Astrochemistry Lecture 3
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Transcript of ISM & Astrochemistry Lecture 3
ISM & AstrochemistryLecture 3
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
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.
Fractional IonisationH2 + 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 ratek1n(H2) = k4n(XH+)n(e)
Zero-order approximation: Assume n(XH+) = n(e)
Fractional IonisationThen, 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
Oxygen ChemistryH3
+ + 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+, .. ,
Oxygen Chemistry
OH is a very reactive radicalO + OH H + O2 M for T > 160K, fast
C + OH H + CON + OH H + NO M for T > 100K, fastS + OH H + SO M at T = 300K, fastSi + 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)
ResultsOxygen 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
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)
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)
Abundance of MethaneH3
+ + 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
Formation of OrganicsStarts 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
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
Formation of OrganicsRadiative 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.