ISM & Astrochemistry Lecture 2

Protoplanetary Nebula The evolutionary stage between evolved stars and planetary nebula CRL 618 – many organic molecules Including the only extra-solar system detection of benzene, C 6 H 6 Time scale of chemistry and evolution of this object is years

Molecule formation in shocks Supersonic shock waves: Sound speed ~ 1 km s -1 Shocks compress and heat the gas Hydrodynamic (J-type) shocks: immediately post-shock, density jumps by 4-6, gas temperature ~ 3000(V S /10 km s -1 ) 2 Gas cools quickly (~ few tens, hundred years) and increases its density further as it cools – path lengths are small. MHD (C-type) shocks: shock front is preceded by a magnetic precursor, gas density and temperature change continuously, ions and neutrals move at different velocities – path lengths are large Importance for chemistry: Endothermic neutral-neutral reactions can occur.

Interstellar and Circumstellar Molecules H2H3+CH3CH4CH3OHCH3NH2HCOOCH3(CH3)2O(CH3)2CO COCH2NH3CH2NHCH3SHCH3CCHCH3C3NC2H5OHCH3C5N CSNH2H3O+H2CCCC2H4CH3CHOHC6HC2H5CNCH3CH2CHO CNH2OH2COc-C3H2CH3CNc-CH2OCH2C7HCH3C4H(CH2OH)2 C2H2SH2CSCH2CNCH3NCCH2CHCNHOCH2CHOC8HHCOOC2H5 CHCCHc-C3HNH2CNCH2CHOHC5NCH3COOHHC7NHC9N CH+HCNl-C3HCH2CONH2CHOC6HH2CCCHCNCH3CONH2CH3C6H HFHNCC2H2HCOOHHC3NH+CH2CHOHH2C6CH3CHCH2C6H6 CF+HCOHCNH+C4HH2CCCCC6H-CH2CHCHOC8H-C3H7CN SiOHCO+H2CNHC3NC5HNH2CH2CNHC11N SiSHOC+HCCNHCCNCHC4HC2H5OCH3 SiCN2H+HNCOHNCCCHC4N SiNHNOHOCNH2COH+c-C3H2O NHHCS+HCNOC4H-CH2CNH NOC3HNCSSiH4C5N- SOC2OHSCNC5C5N SO+C2SC3NSiC4 CPSO2C3OCNCHO PON2OC3S PNCO2C3N- HClH2O+HCO2+ KClH2Cl+CNCHO AlClOCSC-SiC2 OHMgNCAlFAlNCAlOHNaCl OH+MgCNSiNCCCPHCPFeO SHNaCNCO+O2N2 CN-SiCN

One-body reactions Photodissociation/photoionisation: Unshielded photorates in ISM: β 0 = s -1 Within interstellar clouds, characterise extinction of UV photons by the visual extinction, A V, measured in magnitudes, so that: β = β 0 exp(-bA V ) where b is a constant (~ 1- 3) and differs for different molecules

Cosmic Ray Ionisation H 2 + crp → H e - H H 2 → H H He + crp → He + + e - He + + H 2 → products exothermic but unreactive H 3 + : P.A.(H 2 ) very low Proton transfer reactions very efficient Key to synthesising molecules He + : I.P.(He) very large Breaks bonds in reaction Key to destruction of molecules IS Chemistry efficient because He + does not react with H 2

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, cm -3 )

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

Formation of Molecules 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 Radiative association: A + + B → AB + + hν Photon emission more efficient as size of complex grows, therefore can be important in synthesising large molecular ions CH H 2 → CH h ν k(T) = (T/300) -1 cm 3 s -1 CH HCN → CH 3 CNH + + h ν k(T) = (T/300) -0.5 cm 3 s -1

Chemical Kinetics A + B → C + Dk = cm 3 s -1 Loss of A (and B) per unit volume per second is: dn(A)/dt = - kn(A)n(B)cm -3 s -1 where n(A) = no. of molecules of A per unit volume Formation of C (and D) per unit volume per second is: dn(C)/dt = + kn(A)n(B)cm -3 s -1 - Second-order kinetics – rate of formation and loss proportional to the concentration of two reactants

First-order kinetics A + hν → C + Dβ (units s -1 ) Loss of A (and B) per unit volume per second is: dn(A)/dt = - βn(A)cm -3 s -1 where β = photodissociation rate of A (s -1 ) Aside: The number, more accurately, flux of UV photons or cosmic-ray particles, is contained within β or ς - First-order kinetics – rate of formation and loss proportional to the concentration of one reactant

General case dn(X j )/dt = Σ k lm [X l ][X m ] + Σ β n [X n ] - [X j ]{ Σ k jl [X l ] + Σ β j } m -3 s -1 or d[X]/dt = F X – L X [X] Need to solve a system of first-order, non-linear ODEs - solve using GEAR techniques -Steady-state approximation – rate of formation = rate of loss F X = L X [X] ss so that [X] ss = F X /L X Need to solve a system of non-linear algebraic equations - solve using Newton-Raphson methods

Time scales d[X]/dt = F X – L X [X] For simplicity, assume F X and L X are constants and [X] = 0 at t =0 (initial condition) Solution is: [X,t] = (F X /L X ){1 – e -L x t } [X,t] = [X] ss {1 – e -t/tc } where t c = 1/L X Note: As t → ∞, [X] → [X] ss When t = t c, [X,t c ] = 0.63[X] ss, so most molecular evolution occurs within a few times t c

Grain Surface Time-scales Collision time: t c = [v H (πr 2 n d )] -1 ~ 10 9 /n(cm -3 ) years Thermal hopping time:t h = ν 0 -1 exp(E b /kT) Tunnelling time:t t = v 0 -1 exp[(4πa/h)(2mE b ) 1/2 ] Thermal desorption time: t ev = ν 0 -1 exp(E D /kT) Here E b ~ 0.3E D, so hopping time < desorption time For H at 10K, E D = 300K, t t ~ s, t h ~ s Tunnelling time < hopping time only for lightest species (H, D) For O, E D ~ 800K, t h ~ s. For S, E D ~ 1100K, t h ~ 250 s, t t ~ 2 weeks Heavy atoms are immobile compared to H atoms

Formation of H 2 Gas phase association of H atoms far too slow, k ~ cm 3 s -1 Gas and dust well-mixed In low-density gas, H atoms chemisorb and fill all binding sites (10 6 ) 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 H 2

Formation of Molecular Hydrogen Gas-Phase formation: H + H → H 2 + hνvery slow, insignificant in ISM Grain surface formation: Langmuir-Hinshelwood (surface diffusion) Eley-Rideal (direct hit)

Grain Surface Chemistry Zero-order approximation: Since H atoms are much more mobile than heavy atoms, hydrogenation dominates if n(H) > Σ n(X), X = O, C, N Zero-order prediction: Ices should be dominated by the hydrogenation of the most abundant species which can accrete from the gas-phase Accretion time-scale: t ac (X) = (S X v X σn d ) -1, where S X is the sticking coefficient ~ 1 at 10K t ac (yrs) ~ 10 9 /n(cm -3 ) ~ 10 4 – 10 5 yrs in a dark cloud

Interstellar Ices Mostly water ice Substantial components: - CO, CO 2, CH 3 OH Minor components: - HCOOH, CH 4, H 2 CO Ices are layered - CO in polar and non-polar ices Sensitive to f > Solid H 2 O, CO ~ gaseous H 2 O, CO