ISM & Astrochemistry Lecture 3

Models - History – Grain surface chemistry – H 2, CH, CH – Ion-neutral chemistry – HD, DCO – Neutral-neutral chemistry – HC 3 N 2000-date – Gas/Grain interaction – D 2 CO, ND 3 10,000 reactions, 500 species

Dark Clouds H 2 forms on dust grains Ion-neutral chemistry important Time-scales for reaction for molecular ion M + - 1/kn(X) – 10 9 /n(H 2 )for fast reaction with H 2 – 10 6 /n(e)for fast dissociative recombination with electrons – 10 9 /n(X)for fast reaction with X Since n(e) ~ n, dissociative recombination is unimportant for ions which react with H 2 with k > cm 3 s -1 ; Reactions with X are only important if the ion does not react, or reacts very slowly, with H 2 since n(X) = n(H 2 ) at most.

Fractional Ionisation H 2 + crp  H ek 1 - cosmic ray ionisation H H 2  H Hk 2 H X  XH + + H 2 k 3 XH + + e  neutral productsk 4 - dissociative recombination Consider XH+: Steady-state: formation rate = destruction rate k 1 n(H 2 ) = k 4 n(XH + )n(e) Zero-order approximation: Assume n(XH + ) = n(e)

Fractional Ionisation Then, the fractional ionisation, f(e), can be written: f(e) = n(e)/n(H 2 ) = [k 1 /k 4 n(H 2 )] 1/2 Put in rate coefficients: k 1 = s -1, k 4 = cm 3 s -1 Then f(e) = /n 1/2 (H 2 ) i.e. f(e) ~ – for n(H 2 ) ~ cm -3 in dark clouds

Oxygen Chemistry H O  OH + + H 2 M - measured OH + + H 2  H 2 O + + HM H 2 O + + H 2  H 3 O + + HM H 3 O + + e  O, OH, H 2 OM Destruction of H 2 O: He +, C +, H 3 +, HCO +,.. (M) Destruction of OH: He +, C +, H 3 +, HCO +,..,

Oxygen Chemistry OH is a very reactive radical O + OH  H + O 2 M for T > 160K, fast C + OH  H + CO N + OH  H + NOM for T > 100K, fast S + OH  H + SOM at T = 300K, fast Si + OH  H + SiO C + O 2  CO + OM for T > 15K, fast CO is the most abundant IS molecule – after H 2 n(CO) ~ n(H 2 )

Results Oxygen chemistry O 2 abundance to ~ 100 times larger than observed H 2 O abundance close to ~ 100 times larger than observed PROBLEM!! T = 10K, n(H 2 ) = 10 4 cm -3

Carbon Chemistry (diffuse clouds) C + + H 2  CH + + Hendoergic by about 0.4eV (4640K) C + + H 2  CH hnutheory – k~ cm 3 s -1 CH H 2  CH HM – k ~ cm 3 s -1 CH e  productsM – k 1 ~ cm 3 s -1 CH hnu  productsM – k 2 ~ s -1 (unshielded) CH H 2  CH hnuM – k 3 ~ cm 3 s -1 Loss of CH 3 + : k 1 n(e) vs k 2 vs vs k 3 n(H 2 ) n(e) = n(C + ) = n; n(H 2 ) = 0.01n (typically); n ~ 100 cm -3 Loss of CH 3 + : vs vs (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) H C  CH + + H 2 M - measured CH + + H 2  CH HM CH H 2  CH HM CH H 2  CH HEndoergic, but … CH H 2  CH hnuM – slow ( cm 3 s -1 ) CH e  CH, CH 2, CH 3 (mostly), CH 4 M CH CO  CH 4 + HCO + M – dominant loss for CH 5 + Destruction of CH 4 : He +, C +, H 3 +, HCO +,.. (M)

Abundance of Methane H C  … CH 4 k 1 = cm 3 s -1 CH 4 + X +  products k 2 = cm 3 s -1 Destruction of CH 4 : He +, C +, H 3 +, HCO +,.. (M) Steady-state: Formation rate = destruction rate k 1 n(C)n(H 3 + ) = k 2 n(X + )n(CH 4 ) n(CH 4 )/n(C) = n(H 3 + )/n(X + ) ~ 0.1 A significant fraction of C atoms is converted to methane

Formation of Organics Starts with proton transfer from H 3 + C + H 3 +  CH + + H 2 CH + + H 2  CH H CH H 2  CH H CH H 2  CH hυ CH CO  CH 4 + HCO + C + + CH 4  C 2 H H 2 C + + CH 4  C 2 H H

Formation of Hydrocarbon Chains C insertion: C + C m H n +  C m+1 H n H C + + C m H n  C m+1 H n H C + C m H n  C m+1 H n-1 + H Binary reactions: C 2 H + C 2 H 2 +  C 4 H H C 2 H + C 2 H 2  C 4 H 2 + H CN + C 2 H 2  HC 3 N + H Carbon and carbon-bearing molecules are very reactive with each other They are not reactive with H 2, most reactions are endoergic So carbon-chains build easily in cold, dark clouds – as observed

Formation of Organics Radiative association: CH H 2 O  CH 3 OH hnu CH HCN  CH 3 CNH + + hnu CH CH 3 OH  CH 3 OCH hnu Dissociative recombination: C 2 H e -  C 2 H 2 + H CH 3 OH e -  CH 3 OH + H CH 3 OCH e -  CH 3 OCH 3 + H RA reactions occur faster for larger systems – in many cases each collision leads to a product – compare with C + and H 2, where only 1 in 10 7 collisions produces CH 2 + In DR many product channels can occur, the ‘preferred’ channel might actually be a minor channel.