Low-Temperature Gas-Phase & Surface Reactions in Interstellar Clouds ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY Dense Interstellar Cloud Cores 10 K 10(4) cm-3 H2 dominant Molecules seen in IR absorption and radio emission sites of star formation
Cosmic rays create weak plasma Fractional ionization < 10(-7) Cosmic Elemental Abundances H =1 He = 6.3(-2) O = 7.4(-4) 1.8(-4) C = 4.0(-4) 7.3(-5) N = 9.3(-5) 2.1(-5) S = 2.6(-5) 8.0(-8) Si = 3.5(-5) 8.0(-9) Fe = 3.2(-5) 3.0(-9)
Dust/gas = 1% by mass Gas-phase abundances of heavy elements in clouds reduced. GAS PHASE INTERSTELLAR/CIRCUMSTELLAR MOLECULES - HIGH RESOLUTION (12/03) _____________________________________________________________________________________________ H2 KCl HNC C3S C5 HCO
OH 2(-7) NH3 2(-8) HC3N 2(-8) N2H+ 4(-10) HNC 2(-8) O2 < 8(-8) AN INTERSTELLAR GRAIN 0.1 ices silicates & carbonaceous material + small grains and PAHs
Water ice = 10(-4) of Gas density Water, CO, CO2 Formation of Hydrogen H2+ + e H H 2 Cosmic ray dust particle O
H Efficient Low T GasPhaseReactions 1. 2. 3. 4. 5. Ion-molecule reactions Radiative association reactions Dissociative recombination reactions Radical-radical reactions Radical-stable reactions Ea = 0 Exothermic In areas of star formation, reactions with barriers occur.
Ion-Molecule Reactions A B C D Experimental evidence down to a few K Rate coefficients explained by classical capture models in most but not all instances. ion-non polar (Langevin case) 9 k L 2e 2 10
cm3 s-1 Ion-mol. Rx. (cont) Ion-polar kTS k L [0.62 0.4767 x ] 2 k k [1 x] 10 7 cm3 s 1 LD L 1/ 2 D 1/ 2 x T 2k BT + more complex state-specific models
Remaining Questions 1) Why are some reactions slow? H 3 HD H 2 D H 2 2) Is there a quantum limit? 4 2 k 2 ( 2 1 ) sin
Radiative Association * A B AB AB h k1 k ra k r K (T )k r ; k r 102 s 1 k 1 K (T ) T ( rA rB ) / 2 , size, bond engy Few ion trap measurements by Gerlich, Dunn down to 10 K What is the 0 K limit?
What about competitive channels? Dissociative Recombination Reactions AB e A B Studied in storage rings down to zero relative energy; products measured for approx.10 systems k (T ) A(T / 300) n 7 n=0.5, 1.5 3 1 A 10 cm s Some systems studied: H3+, HN2+, HCNH+, H3O+, NH4+, CH5+ ,CnHm+
QUESTION How large must ions be before the dominant process becomes radiative recombination? statistical trapping Answer via statistical theories (RRKM): 20-30 atoms????? Radical-radical Reactions OH + O(3P) O2 + H 1 2 18 0 6 4 2 005010T 15 0
2 0 2 5 0 3 0 /K -11 3 -1 -1 k / 10 cm molec s present experimental results Davidsson & Stenholm, EL, 1990 Clary, ACCSA, 1984 Harding & Troe, cS SACM + CT, 2000 Howard & Smith, 1980 Lewis & W atson, 1980
Howard & Smith, 1981 Stewart & Smith, 1994 Current astro chemical model Robertson & GP Smith, 2002 this work Detailed capture models by Clary, Troe RADICAL-NEUTRAL RX (CONT) CN + C2H2 HCCCN + H YES C + C2H2 C3H + H YE S CCH + HCN HCCCN + H NO
Barrier cannot be guessed!! Attachment e A A h If enough large molecules with large electron affinities present, electrons may not exist! Note no competitive fragmentation channels. FORMATION OF GASEOUS WATER H2 + COSMIC RAYS H2+ + e Elemental abundances: C,O,N =
Solved kinetically; thermodynamics useless! t=0; atoms except for H2 Latest network osu.2003 contains over 300 rapid neutral-neutral reactions. Rate coefficients estimated by Ian Smith and others for many of these. Verification needed!! GAS-PHASE MODELS OF DENSE CLOUD CORES "SUCCESSES" + 1. IONS ( H3 , HCO+, HC3NH+) 2. ISOMERS (HNC) & RADICALS (OH) HCNH+ + e ----> HCN + H; HNC + H 3. ISOTOPIC FRACTIONATION H3+ + HD <====> H2D+ + H2 4. UNSATURATED MOLECULES A+ + H2 -------> No Reaction 5. ORDER-OF-MAGNITUDE AGREEMENT WITH AT BEST 80% OF MOLECULES
Chemistry imperfect!! Nature of Solution for a homogeneous, timeindependent cloud early time if Orich fi Small species (CO) Large species (HC9N) 0.1 10 Time (Myr) Nature of Solution for a homogeneous, timeindependent cloud early time if Orich fi Found in pre-stellar cores
accretion Small species (CO) Large species (HC9N) 0.1 10 Time (Myr) Low Temperature Surface Chemistry on Amorphous Surfaces 1) Mechanisms (diffusive [LangmuirHinshelwood], Eley-Rideal, hot atom, impurity site) 2) Dependence on size, mantle, fluffy nature, energy parameters 3) Rate equations vs. stochastic treatments 4) non-thermal desorption (cosmic rays) Edes Ediff
physisorption (diffusion) Desorption & Diffusion 1 k des ( s ) exp( Edes / k BT ) 0 * for heavies Desorption via evaporation and cosmic-ray heating. 1 k hop ( s ) exp( Ediff / k BT ); Ediff 0.30 Edes kdiff = khop/N; N is the number of binding sites For H, tunneling can occur as well. H diffuses the fastest and dominates the chemistry.
TYPES OF SURFACE REACTIONS REACTANTS: MAINLY MOBILE ATOMS AND RADICALS A + B H + H H2 H +
X WHICH CONVERTS AB association XH (X = O, C, N, CO, etc.) O OH H2O C CH CH2 CH3 CH4 N NH NH2 NH3 CO HCO H2CO H3CO CH3OH X + Y XY
(CO + O CO2) ?????????? Experiments on cold surfaces Vidali et al. Formation of H2 on silicates, carbon, and amorphous ice; LH mechanism characterized and energies obtained; formation of CO2; energy partitioning of hydrogen product (also UCL group) Ediff(H, olivine) = 287 K; Ediff(H, carbon) = 511 K But whole analysis of data has been questioned by others, who feel that both tunneling and some chemisorption sites are involved!!!!! Hiraoka et al. Formation of ices (CH4, H2O,NH3, H2CO) Watanabe et al. Formation of methanol Danish group formation of H2 MODELLING DIFFUSIVE SURFACE CHEMISTRY Rate Equations dN ( H ) k acc n( H ) k des N ( H ) K H H N ( H ) N ( H )
dt K H H k diff ( H ) k diff ( H ) The rate coefficient is obtained by K A B k hop N ( B) / N Method accurate if N>1 Biham et al. 2001 STOCHASTIC METHODS Based on solution of master equation, which is a kinetictype equation in which one calculates not abundances but probabilities that certain numbers of species are present. Can solve directly (Hartquist, Biham) or via Monte Carlo realization (Charnley). MASTER EQUATION dN ( H )
k acc n( H ) k des N ( H ) K H H N ( H ) N ( H ) dt replaced when N(H) << 1 by a series of coupled equations for Pn(H): = n Pn(H) dP0(H)/dt = . Stochastic States Unfortunately, with more than one reactive surface species, one must compute joint probabilities P ( n1 , n2 , n3 ...) so that the computations require significant computing power. It is necessary to impose cutoffs on the ni and the total number of surface species considered. More simple fix: modified rate method
New Gas-Grain StochasticDeterministic Model Stantcheva & Herbst (2004) Gas-phase chemistry solved by deterministic rate equations, while surface chemistry solved by solution of master equation. Some results quite different from total deterministic approach. RESULTS: surfaces From observations of grain mantles, the dominant species in the ice are water, CO, CO2, and occasionally methanol. The models at 10 K and a gas density of 10(4) cm-3 are able to reproduce the high abundance of water, seem to convert CO into methanol too efficiently, and tend to underestimate the amount of CO2. Results sensitive to density. The modified rate method reproduces the master equation approach at 10 K, but the normal rate method can be in error.
Results from Stantcheva & Herbst (2004) CO % Agreement in TMC-1 Gas-phase species Roberts & Herbst 2002 Some Conclusions 1) Low-temperature chemistry in interstellar clouds (both gas-phase and surface) partially understood only. 2) Chemistry gives us many insights into the current state and history of sources 3) More work on cold chemistry is clearly needed to make our mirror into the cosmos more transparent.
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