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1. Introduction

2 Mar 2005, New trend WS. Evolution of CO molecule on dusts in dense core : formation of H 2 CO and CH 3 OH. Naoki Watanabe. Colleagues: A. Nagaoka, T. Shiraki, H. Hidaka, A. Kouchi. Institute of Low Temperature Science, Hokkaido Univ. 1. Introduction. 2. Experimental.

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1. Introduction

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  1. 2 Mar 2005, New trend WS Evolution of CO molecule on dusts in dense core : formation of H2CO and CH3OH Naoki Watanabe Colleagues: A. Nagaoka, T. Shiraki, H. Hidaka, A. Kouchi Institute of Low Temperature Science, Hokkaido Univ. 1. Introduction 2. Experimental 3. Formation of H2CO & CH3OH 4. Deuterium fractionation in CH3OH by the surface reactions 1/22

  2. Ice mantle ~10K (H2O, CO2, CO,…) Gas phase reactions silicate core Surface (solid phase) reactions on dusts 0.1~1mm ・ mainly, ion-molecule reactions necessary for H2, H2O, organic molecules and the formation of ice mantle ・ reaction network models Evolution of molecules in a molecular cloud Molecular cloud ・ Many species ＞120 ・ Ice dusts : ~10-9 cm-3 ・ 10 K < T ? Well studied ! 2/22

  3. Infrared absorption spectra of ice dust (ISO observation) Silicate 102 CH4 H2O OCS 13CO2 Silicate CO2 101 -CH3 CO Flux (Jy) CH3OH XCN Acrobat•¶‘ 100 CO2 W33A H2O 10-1 20 10 5 3 l (mm) 3/22

  4. E l i a s 2 9 W 3 3 A N G C 7 5 3 8 E l i a s 1 6 M o l e c u l e s l o w h i g h I R S 9 / h i g h f i e l d H O 1 0 0 1 0 0 1 0 0 1 0 0 2 C O 9 1 6 5 . 6 2 5 C O 1 4 2 0 2 2 1 5 2 H C O 1.7-7 5 2 C H O H 2 2 5 < 4 < 3 . 4 3 C H 2 2 < 1 . 6 4 N H 1 5 1 3 < 9 . 2 < 6 3 Main components in an ice mantle (Ehrenfruend & Charnley 2000) 4/22

  5. ・　Photolysis H2O(s) + hn → H + OH OH + CO(s) → CO2 + H ・　Cosmic-ray induced reaction H H2O(s) + p → H + OH H + O → OH → H2O OH + CO(s) → CO2 + H ・　Surface reaction of atoms（H, C, O) H + H → H2 on the surfaces fH~105 > fuv~103 cm2/s in dense core Surface (Solid-phase) reactions on ice dusts 5/22

  6. H H H H CO → HCO → H2CO → CH3O → CH3OH Formation of H2CO and CH3OH on dusts Photolysis of the ice mantle CO(s) + H2O(s) + hv (or ion ) →CO + H + OH　→　HCO + OH Inefficient → H2CO ・・・ → CH3OH Successive hydrogenation of CO on the surface ? Eb : 2000 ~ 3000 K >> surface temperature 6/22

  7. ・　Do reactions proceed at the temperature around 10 K ? H H H H CO → HCO → H2CO → CH3O → CH3OH Try ! ☆H + CO-H2O mixed ice :10~20 K H CO H2O ice Aim If those proceed, gain more information ・ Reaction rate, reverse process, … ・　Dependence of reaction rates on ice temperature & composition. 7/22

  8. CO, H2O gas Al substrate ~10K Measuring the IR absorption spectra of ice H Measuring the spectra during H exposure Experimental H Al substrate (10K~) port for H-atom measurements Base pressure: 2×10-10 Torr Sample temp.：8 ~ 20K Sample thickness：<30 ML 8/22

  9. ・ IR absorption spectrum for the initial sample (nonexposed) ～ ～ Å O/CO 4, 100 10K, H 2 0.06 0.04 Absorbance CO 0.02 H O H O 2 2 0.00 4000 3000 2000 1000 Wavenumber (cm-1) 9/22

  10. What’s found H C O 2 t = 1 m i n 1. H2 does not react H C O 2 e c C H O H t = 6 m i n n C O 3 a b r o s b A H H H H t = 4 0 m i n D CO →HCO→ H2CO→CH3O→CH3OH (4) (3) (2) (1) H O 2 0 . 0 0 5 k(1) << k(2), k(3) << k(4) Eb(1), (3) : 2200 ~ 2600 K (Woon, 2002) 1 0 0 0 3 0 0 0 2 0 0 0 - 1 W a v e n u m b e r ( c m ) These are tunneling reaction ・ Variation of the IR absorption spectrum of ice after the H exposure 15 K increase 2. CO → H2CO → CH3OH decrease 3. HCO, CH3O not observed 10/22

  11. ・ Ice temperature dependence of reactions 0 1 2 3 4 5 0.00 CO 10 K 15 K 20 K -0.05 -0.10 -0.15 feature of tunneling reaction 0.15 (CO) H CO 2 0 0.10 (X) / N 0.05 t transition stage between thermal and tunneling reactions N 0.00 0.15 CH OH 3 0.10 0.05 may be due to drop of sticking probability of H atoms at 20 K 0.00 0 20 40 60 80 Exposure Time (min) H fluence (1018 cm-2) 10cm -3, 10 K, 105 yr Features ・ rates of CO decrease and H2CO increase are almost the same between 10 and 15 K ・ rates of CH3OH increase and yield at 10 K < those at 15 K ・ All rates at 20 K are very slow 11/22

  12. Arrhenius + Tunneling H2CO → CH3OH Arrhenius plot for the tunneling reaction Slope：activation energy Arrhenius log k CO →H2CO 15K 10K T-1 12/22

  13. 100 20K, 100Å 10 10K, 100Å NGC7538 Halley H2CO / CH3OH 15K, 100Å GL2136 1 Hale-Bopp W33A 20Å Hyakutake 0.1 Orion hot core GL7009S 10Å 0.01 0.1 1 10 100 1000 CO / CH3OH Observations vs. Experiments Experimental fluences = those for 106 yr in MC 13/22

  14. ・　The CO hydrogenation proceeds efficiently under the condition of MC H H H H CO → HCO → H2CO → CH3O → CH3OH (4) (3) (1) (2) Summary 1 ・ These are tunneling reactions ・　k(1) << k(2), k(3) << k(4) ・　Reactivity strongly depends on the temperature of the surface. 14/22

  15. ［D］/［H］ ratio in methanol CD3OH /CH3OH CH3OD /CH3OH CH2DOH /CH3OH CHD2OH /CH3OH Molecular cloud (IRAS16293) 0.01 0.06 0.3 0.02 Comets <0.008 <0.03 Interstellar atomic D/H ratio ～ 1.6 × 10-5(Linsky et al. 1995) 15/22

  16. H3+ + HD H2D+ + H2 ? ×multi-deuterated methanol Models for the deuterium fractionation in methanol Gas phase models HD/H2~10-5 (initial condition: cosmic ratio） H2D+/H3+ >> HD/H2 ~10-5 producing methanol-d in gas phase Gas - dusts models H2D+ + e → H2 + D D / H atom ~ 0.1 >> HD/H2 after 104 yr methanol-d D + CO on a surface 16/22

  17. H D H D e. g., CO → HCO → HDCO → CHD2O → CHD2OH D D e. g., CH3OH → CH2DOH → CHD2OH H H Try ! CH3OH + D atom Deuterium fractionation in methanol by surface reactions Process 1. (previous models) Successive H and D addition to CO Slow ? … D ≧ H addition required Process 2. (our idea) H-D substitution in methanol after the formation of CH3OH 17/22

  18. H-D substitution (process 2) proceeds ! CH3OH + D → dn-methanol but dn-methanol + H → CH3OH not observed ! D + CH3OH at 10 K CO-stretch Initial absorbance of CH3OH OH-stretch Absorbance CH3-deformation dn-CH3OH 150-min exposure to D increase D Absorbance decrease 18/22

  19. Process 1 (successive D and H addition) V.S. Process 2 (H-D substitution in methanol) Which is dominant in MC ? Try H and D atom + solid CO ! in which the two processes compete. 19/22

  20. initial sample Results H + D + CO at 10 K 0.005 （D/H=0.1) after exposure 20/22

  21. 106 yr × Variation of CO, CH3OH and CH3OH-d Successive addition e. g., CO → HCO → HDCO → CHD2O → CHD2OH CO → DCO → D2CO → CD3O → CD3OH H-D substitution e. g., CH3OH → CH2DOH → CHD2OH 21/22

  22. Summary 2 ・　H - D substitution reaction in solid methanol is key process for the deuterium enrichment ! Future work ・ Which is dominant ? H abstraction & D addition d3 - CH3O d2 - CH3O CH3O d1 - CH3O -H -H -H -H +D +D +D +D d4- CH3OH d3 - CH3OH d1 - CH3OH d2 - CH3OH CH3OH H-D direct exchange Observed isotopomer 22/22

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