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低温表面原子反応による星間分子の 生成機構 有機分子生成,重水素濃集 渡部直樹,長岡明宏,白木隆弘, 日高 宏 ,香内 晃
ALMA 研究会 低温表面原子反応による星間分子の 生成機構 有機分子生成,重水素濃集 Today, I would like to emphasize the importance of successive hydrogenation of CO for the origin of cometary formaldehyde and methanol. 渡部直樹,長岡明宏,白木隆弘, 日高 宏 ,香内 晃 北海道大学・低温科学研究所
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星間塵表面反応の重要性 1.星間塵は彗星・惑星系の原材料物質 2.気相での生成が難しい分子種
H2, H2O, H2CO, CH3OH, 複雑な有機物など 3.星間塵マントルの存在(気相からの吸着では説明できない) 星間塵表面反応は分子進化のKey process!
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Chemical reaction on icy particle
This illustration summarize the chemical reactions occurred on the surface and in the ice. CO is formed in the gas phase. CO2 is formed by the photochemical reactions: Water ice is decomposed by UV to OH+H. This OH react with CO and results in the formation of CO2. H2 molecules are formed by surface reaction as already shown by Dr. Takahashi. Water ice might be formed by the combination of H and O atoms on the particle surface. Organic molecules, such as formaldehyde, is also formed by the surface reactions as shown here. CO 0.1mm H2O,CO,CO2,H2CO,CH3OH,NH3
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Composition of icy mantles
h r e n f r e u n d & C h a r n l e y 2 W 3 3 A N G C 7 5 3 8 E l i a s 2 9 E l i a s 1 6 M o l e c u l e s h i g h I R S 9 / h i g h l o w f i e l d Surface reactions C H 4 2 < 1 . 6 3 O 5 N 9 1.7-7 H O 1 C O 9 1 6 5 . 2 gas phase C O 9 1 6 5 . 2 C O 9 1 6 5 . 6 2 5 C O 2 1 4 5 UV C O 2 1 4 5 C O 2 1 4 5 C H 4 2 < 1 . 6 3 O 5 N 9 1.7-7 This table is an example of composition of amorphous ice in various molecular clouds. Major component is water ice, and other molecules are smaller than 25%. Among these molecules, the formation mechanism of CO is understood by the gas phase reactions. CO2 is formed by the photochemical reactions in the icy mantle. In theoretical studies, it is assumed that other molecules are formed by the surface reactions on the solid particles. However, there is no experimental evidence of the surface reactions.
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H2O-CO アモルファス氷上における水素原子付加反応によるH2CO, CH3OH分子の生成実験
Composition of icy mantles H2O-CO アモルファス氷上における水素原子付加反応によるH2CO, CH3OH分子の生成実験 E h r e n f r e u n d & C h a r n l e y 2 W 3 3 A N G C 7 5 3 8 E l i a s 2 9 E l i a s 1 6 M o l e c u l e s Naoki Watanabe & Akira Kouchi h i g h I R S 9 / h i g h l o w f i e l d Astrophys. J. Lett. 571, 173 (2002). H O 1 2 C C O O 9 9 1. 背景 1 1 6 6 5 5 . . 6 6 2 2 5 5 2. 実験装置 C C O O 1 1 4 4 2 2 2 2 2 2 1 1 5 5 2 2 C H 4 2 < 1 . 6 3 O 5 N 9 1.7-7 H O 1.7-7 5 C 2 We focus our attention on CO, H2CO and CH3OH, because CO is the most primitive molecules in molecular clouds, and because these two molecules are assumed to be formed by the H-atom addition to CO. 3. 結果と議論 C H O H 2 2 4. まとめ 5 < 4 < 3 . 4 3 C H 2 2 < 1 . 6 4 N H 1 5 1 3 < 9 . 2 < 6 3
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Successive hydrogenation of CO
CO HCO H2CO CH3O CH3OH ・large abundance in ice ・could not explained by gas phase reaction ・could not explained by UV reactions in ice (Schutte et al. 1993) Hiraoka et al. (1994, 2002) H-addition experiment onto pure CO at K by Temperature-Programmed Desorption spectra We realize the importance of the successive hydrogenation of CO because: these molecules are abundant in icy grains, formation of these molecules could not explained by gas phase reactions and photochemical reactions in ice. While, Hiraoka et al. made pioneering experiments on the H-atom addition to pure CO at K. They analyzed reaction products by TPD spectra, and found the formation of H2CO with very low yields of <0. 1%, and found no CH3OH. However, there are some problems on their experiments: First, they did not measure flux of H-atoms. Second, they did not observe composition change during H-atoms irradiation. Third, they did not obtain any information on reaction rates. Therefore, it is highly desirable to perform quantitative experiments on the successive reaction. Formation of H2CO(yield < 0.1%) , no CH3OH Problem: H-flux ? TPD Reaction rate ? Need for the quantitative experiments
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Purpose of our series of experiments
Quantitative experiments: ・ measurement of H-flux (new H-source) ・ in-situ observation during H-addition at 8-20 K Questions: ・Does reaction (CO+H) proceed? ・Compositional dependence? ・Is the reaction effective in the molecular cloud? Yes. Watanabe & Kouchi (2002), Watanabe et al. (2003) H2O-CO mixture So, purpose of our series of exeperiments is: We must perform quqntitqtive experiments by measuring H-flux and by in-situ observation during H-atom irradiation. From these experiments, we would like to answer these questions: Does reaction proceed? How effective? and Is effective in MC? Pure CO & H2O-CO mixture Watanabe et al.(2004) Yes. Rate constants
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COの存在形態 赤色:CO分子 星間塵 星間塵 CO-H2O well mixed マントル 純CO固体+H2O ice
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Experimental set-up H2O+CO, CO Temperature of H (30,80,300 K)
This is a schematic illustration of our experimental setup, which enables us to realize the conditions of molecular clouds in the laboratory. Al substrate is cooled by a closed cycle He refrigerator to 10 K. Ice is deposited onto this substrate by the introduction of gas mixture of water and CO. This is a newly developed atomic source which will be explained in the next slide. Infrared spectra of ice before and during irradiation of H-atoms are monitored in-situ using FTIR and MCT detector. In the case of H2CO experiments, instead of H2O and CO gas, we introduced H2CO gas by the decomposition of paraformaldehyde at 59 deg C. Temperature of H (30,80,300 K) Port for H-flux measurement by QMS
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Newly developed H-atom source
flux 〜 1015 H cm-2 s-1 Specially designed microwave radiator for large H-flux (McCullough et al., 1993) PTFE-tubes (to prevent recombination) TMP Plasma H (30-300K) H2 Cu-tube Pyrex glass (to prevent recombination) This figure shows the newly developed H-atom source. We used specially designed microwave radiator developed by McCullough et al. to obtain large H flux. Pyrex discharge tube is used to prevent recombination. The Pyrex tube has a snake-like nose to eliminate UV and ions. Furthermore, graphite is coated on this part to eliminate UV. For the transportation of H-atoms, we used PTFE tubes to prevent recombination. PTFE tube is cooled by Cu-block and tube which are cooled by closed cycle He refrigerator. We used deflector with 100 V/cm to filter out ions and quench the 2s-metastable H. By these configuration, atomic H can be cooled to 30 K. shutter TMP Cu-block (20K) for cooling of H Deflector: 100 V/cm graphite coated snakelike nose (to eliminate ions and UV) (to filter out ions and quench the 2s-metastable H)
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LASSIE =LAboratory Set-up for Surface reaction in Interstellar Environment
He-refrigerator MCT FTIR QMS This is a experimental setup called LASSIE. This part is vacuum chamber. We used two closed cycle He refrigerators, one is for the cooling of the substrate, and another for cooling of Atomic H. This part is atomic source. This is a FTIR and this MCT detector. H-source
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ASURA=Apparatus of SUrface Reaction for Astrophysics
He-refrigerator FTIR We also use another set-up called ASURA. atom source
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Experimental procedure
H-atom (30 K) (〜1015 H cm-2 s-1) 4 3 2 . 6 H O C 1 Wavenumber (cm-1) Absorbance Infrared absorption spectrum of the initial H2O-CO ice FTIR MCT H2O+CO,CO,H2CO Al substrate 8-20 K Next, I’d like to explain experimental method. First, Al substrate is cooled to 8-20 K by He-refrigerator. By introducing the gas mixture of water and CO, amorphous ice is deposited on the substrate. Then, H-atoms are irradiated. Before and during the irradiation, we measured IR spectra. This is an example of IR spectra before irradiation. 1-2 x Torr
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Results and discussion: CO→H2CO→CH3OH
Change of spectra during H-irradiation onto H2O-CO ice increase decrease CO → HCO → H2CO → CH3O → CH3OH k(+1) k(+2) k(+3) k(+4) This figure shows the changes in absorption spectra after H-irradiation at 15K, which represent the absorbance variations, deruta Abs, from the initial spectrum of ice. Peaks appearing above and below the baseline indicate an increase and decrease in absorbance, respectively. As CO decrease, H2CO appears immediately. While, methanol increase at a slower rate than H2CO. We did not observe any intermediate radicals, such as HCO and CH3O. This clearly shows that k+1 is smaller than k+2, and k+3 is smaller than k+4. No HCO and CH3O → 15K k(+1)<k(+2), k(+3)<k(+4)
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30K H onto Pure CO H2O-CO mixture 15K 8K 8K 12K 15K 10K 12K
These figures show the experimental results on cold H atom irradiation onto pure CO and H2O-CO mixture. In the case of mixed ice, there is no difference between 8 and 15K. On the other hand, in the case od pure CO, behavior of curves differs very much. At T=8 and 10K, these curves are almost the same as miced ice. At 12K, no reaction occurred at initial few minutes. But CO decreased sudeenly like this. Finally, amount of CO decrease, and production of H2CO & CH3OH are largest among these experiments. In any case, these result clearly show that the succesive hydrogenation of CO proceed very efficiently not only mixed ice but also pure CO.
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10-20% Pure CO H2O-CO mixture Yields of H2CO and CH3OH 15K 8K 8K 12K
Another important fact is that the yields of formalydehyde and methanol are about 10-20%.
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Temperature dependence of effective reaction rate
CO-H2O mixture (CO→H2CO) 3 pure CO (CO→H2CO) 2 Keff ∝ k a(T) (arbitray unit) Pure H2CO (H2CO→CH3OH) 1 OK, let us discuss the temperature dependence of reaction rates. From present experiment, we could not obtain rate constant, k, but effective reaction rate, Keff, which means rate constant x sticking probability. This curve shows the temperature dependence of Keff of the reaction CO-->to H2CO when H atoms are irradiated onto CO-H2O mixture ice. At temperatures between 8 and 15 K, Keff is almost constant. However, at T between 15 and 20, Keff decreases suddenly. This curve is Keff for pure CO. At T=8,10K, Keff is constant and the same as CO-H2O ice. But, decreases suddenly between 10 and 12K. These behavior reflect the temperature dependences of sticking probabilty alfa. H atoms easily stick onto H2O-CO ice but pure CO. In the case of H2CO to CH3OH, Keff do not depend T, and is about half of CO to H2CO. If we could measure the sticking probability as a function of T, we can determine the real rate constants. (K) a(T): sticking probability, k: rate constant
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反応速度(Keff)∝反応速度定数(k)×吸着係数(a(T))
3 CO-H2O Keff 2 CO CO 1 8 10 12 15 20 a(T) :10-12Kで急激に落ちる 表面のCO CO a(T) :15-20Kで急激に落ちる H2O H2O H2Oに触媒効果?
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Schutte UV This figure shows the relation between H2CO/CH3OH vs. CO/CH3OH. Close symbols show experimental results on Mixed ice, open symbols pure CO. Crosses are composition of comets. Plus signs high mass protostars. It is very difficult to explain observation results by Schtte’s experimental result. On the other hand, our results of mixed ice is here, and pure CO is here. An agreement between both results are clear. We conclude that cometary H2CO and CH3OH are products of hydrogenation of CO in the molecular cloud.
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星間分子雲中(原始星)の重水素(D)を含む分子(D体)
HD HDO HDS D2S HDCS DCN DNC NH2D NHD2 ND3 DC3N DC5N N2D+ DCO+ CH2DCN C4D C2D CH2DCCH H2D+ CH3CCD HDCO D2CO CH2DOH CHD2OH CH3OD CD3OH
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103-104 倍Dがメタノールに濃集 星間分子雲中(原始星)の重水素(D)を含む分子(D体) HD 星間塵表面で生成される分子
HDO HDS D2S HDCS DCN DNC NH2D NHD2 ND3 DC3N DC5N N2D+ DCO+ CH2DCN C4D C2D CH2DCCH H2D+ CH3CCD HDCO D2CO CH2DOH CHD2OH CH3OD CD3OH 星間塵表面で生成される分子 0.9a a a 0.014b IRAS16293の観測結果 a Parise et al. 2002 b Parise et al. 2004 星間空間での[D原子]/[H原子] ~ 1.6 × 10-5 (Linsky et al. 1995) 倍Dがメタノールに濃集
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彗星(ガス) 星間塵表面で生成される分子 (D/H) ・分子種ごとに桁で異なる ・星間塵表面で生成される分子が怪しい
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これまでの星間分子の重水素濃集モデル 精神:なんとか表面反応無しでやりたい 気相反応だけで
HD/H2~10-5 (初期条件: cosmic ratio) H3+ + HD → H2D+ + H2 (逆過程は遅い) H2D+/H3+>>HD/H2 あとはイオン-分子反応で濃集させる H2D+ + e → H2 + D D atom/ H atom >> HD/H2 after 104 yr D + X on a surface 表面反応も適当に仮定して入れてみるか イオン-分子反応 精神:なんとか表面反応無しでやりたい
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kH + CO = kD + CO 等々.→ほんとか!
・ D原子/H原子: 0.01~0.1は達成できるが,DX/HX は一桁以上足りない. ・ 多重重水素体(D2CO, CD3OH)を作れない ・ 表面反応の活性化エネルギー,反応速度をH,D 反応で一定(大胆な仮定). kH + CO = kD + CO 等々.→ほんとか! 表面反応を取り入れたモデルの問題点 表面反応をちゃんと考えるべきだろう
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H2CO, CH3OH重水素濃集プロセスのアイデア
気相で D原子/H原子:0.01~0.1を実現したならば 星間塵表面において, ・ COへのD原子付加がH原子付加に比べて速い? ・ H2CO, CH3OHが出来た後HとDが入れ替わる?
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原子結合反応における同位体分別(D/H)
5 10 100 ±
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HとDの付加速度の差? +H +D 5 10 100 ±
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D原子照射による氷組成の時間変化 CO-H2O 混合氷 (10 K) 増加 Base line 減少 Peak assignment:
CO : 2142 cm-1 C-O stretching D2CO : 1696 cm-1 C-O stretching CD3OD : cm-1 C-O stretching 1067 cm-1 1102 cm-1 1124 cm-1 CD3 sym. bending 2097 cm-1 CD3 sym. stretching 2215 cm-1 CD3 asym. stretching CD3 asym. bending
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H原子照射実験との比較 D原子照射 H原子照射 20 K 10 K 15 K CO D2CO H2CO CH3OH CD3OD
D/H = 0.1 (H原子1個/ccに対して) 分子雲年齢106年 10 K 15 K 20 K D2CO H2CO CD3OD CH3OH CO H irradiation data : Watanabe et al. ApJL (2003)
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付加反応のD原子濃集への寄与 D2CO/H2CO : 3% 1.2% 0% CD3OD/CH3OH : 氷温度 10K 15K 20K
3% % % 観測(IRAS16293): ~ 10 % DCO D2CO HDCO +H +D (D/H=0.1) CO 実際はH原子と競合して付加が生じるために,もっとD体の割合は小さい. CD3OD/CH3OH : 付加反応ではCD3ODはほとんど生成されない.
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我々の提案する星間塵表面反応によるH2CO, CH3OH重水素濃集経路
日高らの実験結果 COへのD逐次付加反応では dn-CH3OHは生成されにくい CO CH3OH生成後の置換反応 H付加反応 + D + H DCO HCO × D H D H D2CO HDCO H2CO D H D H D H Hidakaが調べた経路 Nagaokaが調べた経路 新しさ d3 - CH3O d2 - CH3O d1 - CH3O CH3O D H D H D H D H d4- CH3OH d3 - CH3OH d2 - CH3OH d1 - CH3OH CH3OH
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CH3OHからのD体生成は効率的に起こるか?
d1 - CH3OH d1 - CH3O d2 - CH3OH d2 - CH3O d3 - CH3OH d4- CH3OH d3 - CH3O H D 付加 引き抜き -H +D 1) Hの引き抜き + Dの付加反応 H-D交換反応 2) H-D交換反応 CH3OHからのD体生成は効率的に起こるか? D体生成経路は? 目的
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× 実験内容 (2×10-10 Torr, 基板温度10 K) ① CH3OHsolid + D原子照射
② dn-CH3OHsolid + H原子照射 反応が見られなかった × CH3OH CH3O d1 - CH3OH d1 - CH3O d2 - CH3OH d2 - CH3O d3 - CH3OH d4- CH3OH d3 - CH3O H D 付加 引き抜き -H +D add the schematic figures H-D交換反応
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実験結果 CH3OH (10K, 4ML) + D原子照射 増加 減少 CX3ODは生成されず
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実験結果 CH3OH (10K, 4ML) + D原子照射 106 yr
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CH3OHsolid + H, D同時照射実験を行った場合
d2 - CH3O d1 - CH3O CH3O +D -H +D -H +D -H CD3OH CHD2OH CH2DOH CH3OH もし, 1) Hの引き抜き + Dの付加反応でD体が生成されるなら… CH3OHsolid + H, D同時照射実験を行った場合
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CH3OHsolid + H, D同時照射実験を行った場合
d2 - CH3O d1 - CH3O CH3O H-D交換反応 2) H-D交換反応によってD体が生成される D体の生成速度: (H, D原子照射) << (D原子のみ照射) +H +D -H +D -H +D -H CD3OH CHD2OH CH2DOH CH3OH もし, 1) Hの引き抜き + Dの付加反応でD体が生成されるなら… CH3OHsolid + H, D同時照射実験を行った場合 しかし, D体の生成速度: (H, D原子照射) = (D原子のみ照射)
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今後の研究 付加反応 2H + O→H2O, 3H + N→NH3 2D + O→D2O, 3D + N→ND3 (2個の原子源が必要)
交換反応 H2CO + D, NH3 + D, CH4 + D, H2O + D (C-H, N-H, O-Hの結合エネルギーは異なる →交換反応速度も異なる?)
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