ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY

1. Low-Temperature Gas-Phase & Surface Reactions in Interstellar Clouds ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY

2. Dense Interstellar Cloud Cores 10 K 10(4) cm-3 Molecules seen in IR absorption and radio emission H2 dominant sites of star formation Cosmic rays create weak plasma Fractional ionization < 10(-7)

3. 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. Cosmic Elemental Abundances

5. CO 1(-4) HCN 2(-8) C4H 9(-8) HCO+ 8(-9) c-C3H2 1(-8) HC9N 5(-10) OH 2(-7) NH3 2(-8) HC3N 2(-8) N2H+ 4(-10) HNC 2(-8) O2 < 8(-8) Some Fractional Abundances in TMC-1

6. Water, CO, CO2 + small grains and PAH’s Water ice = 10(-4) of Gas density

7. H2+ + e Cosmic ray O

8. Efficient Low T Gas-PhaseReactions • 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.

9. 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) Ion-Molecule Reactions cm3 s-1

10. Ion-polar Ion-mol. Rx. (cont) + more complex state-specific models

11. Remaining Questions 1) Why are some reactions slow? 2) Is there a quantum limit?

12. Radiative Association , size, bond engy Few ion trap measurements by Gerlich, Dunn down to 10 K What is the 0 K limit? What about competitive channels?

13. Dissociative Recombination Reactions Studied in storage rings down to “zero” relative energy; products measuredfor approx.10 systems n=0.5, 1.5 Some systems studied: H3+, HN2+, HCNH+, H3O+, NH4+, CH5+ ,CnHm+

14. QUESTION • How large must ions be before the dominant process becomes radiative recombination? “statistical trapping” • Answer via statistical theories (RRKM): 20-30 atoms?????

15. Radical-radical Reactions Detailed capture models by Clary, Troe

16. RADICAL-NEUTRAL RX (CONT) CN + C2H2 HCCCN + H YES C + C2H2 C3H + H YES CCH + HCN  HCCCN + H NO Barrier cannot be guessed!!

17. Attachment If enough large molecules with large electron affinities present, electrons may not exist! Note no competitive fragmentation channels.

18. FORMATION OF GASEOUS WATER H2 + COSMIC RAYS  H2+ + e Elemental abundances: C,O,N = 10(-4); C<O Elemental abundances: C,O,N = 10(-4); C<O H2+ + H2 H3+ + H H3+ + O  OH+ + H2 OHn+ + H2  OHn+1+ + H H3O+ + e  H2O + H; OH + 2H, etc

19. FORMATION OF HYDROCARBONS H3+ + C  CH+ + H2 CHn+ + H2  CHn+1+ + H; n=1,2 CH3+ + H2  CH5+ + hn CH5+ + e  CH4 + H (5%)  CH3 + 2H (70%) CH5+ + CO  CH4 + HCO+

20. 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!!

21. Chemistry imperfect!!

22. Nature of Solution for a homogeneous, time-independent cloud “early time if O- rich” fi Small species (CO) Large species (HC9N) 0.1 10 Time (Myr)

23. Nature of Solution for a homogeneous, time-independent cloud “early time if O- rich” fi Found in pre-stellar cores accretion Small species (CO) Large species (HC9N) 0.1 10 Time (Myr)

24. Low Temperature Surface Chemistry on Amorphous Surfaces • 1) Mechanisms (diffusive [Langmuir-Hinshelwood], 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)

25. Edes Ediff “physisorption” (diffusion)

26. Desorption & Diffusion for heavies Desorption via evaporation and cosmic-ray heating. 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.

27. TYPES OF SURFACE REACTIONS REACTANTS: MAINLY MOBILE ATOMS AND RADICALS A + B AB association H + H H2 H + X XH (X = O, C, N, CO, etc.) WHICH CONVERTS O  OH  H2O C CH  CH2  CH3  CH4 N  NH  NH2  NH3 CO  HCO  H2CO  H3CO  CH3OH X + Y XY (CO + O  CO2) ??????????

28. 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

29. MODELLING DIFFUSIVE SURFACE CHEMISTRY Rate Equations The rate coefficient is obtained by Method accurate if N>1 Biham et al. 2001

30. STOCHASTIC METHODS Based on solution of master equation, which is a kinetic-type 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).

31. MASTER EQUATION

32. Unfortunately, with more than one reactive surface species, one must compute joint probabilities Stochastic States 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

33. New Gas-Grain Stochastic-Deterministic 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.

34. 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.

35. Results from Stantcheva & Herbst (2004)

36. CO

37. % Agreement in TMC-1 Gas-phase species Roberts & Herbst 2002

38. 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.