1. Introduction2. Experimental2 Mar 2005, New trend WS3. Formation of H2CO & CH3OHEvolution of CO molecule on dusts in dense core : formation of H2CO and CH3OH4. Deuterium fractionation in CH3OH by the surface reactionsNaoki Watanabe Institute of Low Temperature Science, Hokkaido Univ. Colleagues: A. Nagaoka, T. Shiraki, H. Hidaka, A. Kouchi1/22

Evolution of molecules in a molecular cloudMolecular cloud Many species 120 Ice dusts : ~10-9 cm-3 10 K < T Well studied !?2/22

3/22

Main components in an ice mantle(Ehrenfruend & Charnley 2000)4/22

Surface (Solid-phase) reactions on ice dusts5/22

Formation of H2CO and CH3OH on dustsCO(s) + H2O(s) + hv (or ion ) CO + H + OHHCO + OH H2CO CH3OHPhotolysis of the ice mantleSuccessive hydrogenation of CO on the surfaceInefficientEb : 2000 ~ 3000 K >> surface temperature?6/22

AimDependence of reaction rates on ice temperature & composition.If those proceed, gain more information Reaction rate, reverse process, 7/22

Al substrate(10K~)Base pressure: 210-10 TorrExperimentalSample temp.8 ~ 20K Sample thickness

40003000200010000.000.020.040.06H2OCOH2OAbsorbanceWavenumber (cm-1) 10K, H2O/CO4, 100 IR absorption spectrum for the initial sample (nonexposed)9/22

3. HCO, CH3O not observed2. CO H2CO CH3OH Variation of the IR absorption spectrum of ice after the H exposureincreasedecrease15 K10/22

H fluence (1018 cm-2)10cm -3, 10 K, 105 yrFeatures rates of CO decrease and H2CO increase are almost the same between 10 and 15 K

rates of CH3OH increase and yieldat 10 K < those at 15 K

All rates at 20 K are very slow

Ice temperature dependence of reactions11/22

log kT-1Arrhenius plot for the tunneling reactionSlopeactivation energyArrhenius15K10KCO H2CO12/22

Observations vs. ExperimentsExperimental fluences = those for 106 yr in MC13/22

Summary 1Reactivity strongly depends on the temperature of the surface.k(1)

D/H ratio in methanol

Comets0.020.060.3

Molecular cloud(IRAS16293)CH3OD /CH3OHCHD2OH /CH3OHCH2DOH /CH3OHInterstellar atomic D/H ratio 1.6 10-5 (Linsky et al. 1995)CD3OH /CH3OH0.01

Models for the deuterium fractionation in methanolHD/H2~10-5 (initial condition: cosmic ratioH2D+/H3+ >> HD/H2 ~10-5Gas phase modelsH2D+ + e H2 + D D / H atom ~ 0.1 >> HD/H2 after 104 yrD + CO on a surfaceGas - dusts modelsproducing methanol-d in gas phase methanol-d16/22

Deuterium fractionation in methanol by surface reactionsSuccessive H and D addition to COH-D substitution in methanol after the formation of CH3OHProcess 1. (previous models)Process 2. (our idea)Slow ? 17/22D H addition required

AbsorbanceD AbsorbanceInitial absorbance of CH3OHCO-stretchCH3-deformationOH-stretchincreasedecrease150-min exposure to Ddn-CH3OHD + CH3OH at 10 K18/22

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

initial sampleafter exposure0.005Results H + D + CO at 10 K20/22D/H=0.1)

Variation of CO, CH3OH and CH3OH-de. g., CO HCO HDCO CHD2O CHD2OH CO DCO D2CO CD3O CD3OH

Successive additionH-D substitutione. g., CH3OH CH2DOH CHD2OH21/22

Summary 2H - D substitution reaction in solid methanol is key process for the deuterium enrichment ! Future work Which is dominant ?CH3OHCH3Od1 - CH3OHd1 - CH3Od2 - CH3OHd2 - CH3Od3 - CH3OHd4- CH3OHd3 - CH3O-H+D-H+D-H+D-H+DH-D direct exchangeObserved isotopomerH abstraction & D addition22/22

Low-mass protostarReference