formation of H2CO and CH3OH 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

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

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

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

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

? Inefficient 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 ? CO → HCO → H2CO → CH3O → CH3OH H Eb : 2000 ~ 3000 K >> surface temperature 6/22

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

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

・ IR absorption spectrum for the initial sample (nonexposed) 10K, H O/CO ～ 4, ～ 100 Å 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

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

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

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

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

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

［D］/［H］ ratio in methanol CH2DOH /CH3OH CHD2OH /CH3OH CH3OD /CH3OH CD3OH /CH3OH Molecular cloud (IRAS16293) 0.3 0.06 0.02 0.01 Comets <0.008 <0.03 Low-mass protostar 気相の比であることに注意 Reference をいれよう Interstellar atomic D/H ratio ～ 1.6 × 10-5 (Linsky et al. 1995) 15/22

? Models for the deuterium fractionation in methanol Gas phase models HD/H2~10-5 (initial condition: cosmic ratio） H3+ + HD H2D+ + H2 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 D + CO on a surface methanol-d ? ×multi-deuterated methanol 16/22

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

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

Try H and D atom + solid CO ! in which the two processes compete. 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

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

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

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 d1 - CH3O CH3O +D -H +D -H +D -H +D -H d4- CH3OH d3 - CH3OH d2 - CH3OH d1 - CH3OH CH3OH H-D direct exchange Observed isotopomer 22/22