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.