1. 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

2. 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

3. 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

4. Main components in an ice mantleu 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

5. 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

6. ? 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

7. 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

8. 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

9. ・ 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

10. 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

11. ・ 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

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

13. 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

14. 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

15. ［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 × (Linsky et al. 1995)
15/22

16. ? 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

17. 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

18. 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

19. 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

20. 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

21. × 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

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