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Testing grain surface chemistry models using deuterated probes in low-mass star-forming regions.

Testing grain surface chemistry models using deuterated probes in low-mass star-forming regions. Bérengère Parise. Bonn, Germany. Introduction. For 30 years, observations in the ISM have shown fractionations XD/XH  [D]/[H] (~1.5 10 -5 ). D 2 CO/H 2 CO ~ 0.003 in Orion

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Testing grain surface chemistry models using deuterated probes in low-mass star-forming regions.

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  1. Testing grain surface chemistry modelsusing deuterated probes in low-mass star-forming regions. Bérengère Parise Bonn, Germany

  2. Introduction For 30 years, observations in the ISM have shown fractionations XD/XH  [D]/[H] (~1.5 10-5) D2CO/H2CO ~ 0.003 in Orion (Turner, 1990, ApJ 362, L29)  Active grain chemistry D2CO/H2CO ~ 0.05 towards the low-mass protostar IRAS16293 (Ceccarelli et al. 1998, A&A 338, L43) (figure : M.R. Hogerheijde in van Dishoeck & Blake 1998)

  3. Chemical processes Envelope heated by the protostar Prestellar core phase Grain surface reactions ? Gas phase reactions ? Desorption (e.g. Roberts & Millar 2000, A&A 361, 398) (e.g. Tielens 1983, A&A 119, 177)

  4. Gas phase versus grain chemistry Roberts & Millar 2000, A&A 361, 398 Roberts & Millar 2000, A&A 364, 780 Tielens & Hagen, 1982, A&A 114, 245 Tielens, 1983, A&A 119, 177 root reaction : H3+ + HDH2D+ + H2 H2CO Required physical conditions low temperature CO depletion CO Fractionation due to : • enhanced atomic D/H ratio in the gas phase (D formed from H2D+)  requires CO depletion and low temperature • lower activation barriers for reactions involving D H2D+ propagates the deuterium to other molecules Adsorption in grain mantles

  5. A test for surface chemistry ? Charnley et al. 1997, AJ 482, L203

  6. Deuterated methanol in IRAS16293 Parise et al. 2002, A&A 393, L49 Parise et al. 2004, A&A, 416, 159 IRAM 30m observations 23 CH2DOH lines 6 CH3OD lines 15 CHD2OH lines 12 CD3OH lines

  7. Population diagrams IRAS 16293-2422 +3 +38 f(CH2DOH) = 37 % -19 +2.2 f(CH3OD) = 1.8 % -1.2 +8.4 f(CHD2OH) = 7.4 % -4.4 +1.0 f(CD3OH) = 1.0 % -0.6 -2 -4 -4

  8. Comparison to grain chemistry models • requires an atomic D/H ratio ~ 0.1-0.2 in the gas phase. This atomic fractionation is now • reproduced by new generation • models including D2H+ and D3+ • (e.g. Roberts et al. 2003) • See poster by Vastel et al. • CH2DOH/CH3OD very high compared with the statistical ratio 3 CH3OD destroyed in the gas phase by protonation ? CH3ODH+ CH3OH + D + e- CH3OD + H + e- confirmed by Osamura et al. 2004 dashed lines : surface chemistry model Stantcheva et al. 2003, MNRAS 340, 983 red : IRAS16293-2422 observations

  9. What about looking directly on the grains ? • Observations in solid phase are less sensitive than gas phase observations. • H2O : main constituent of icy mantles around dust grains. Search for HDO in the ices Previous attempts : Detection of HDO in high mass protostars W33A et NGC7538 IRS9(Teixeira et al., 1999, A&A, L19-L22) challenged by Dartois et al. (on the edge of one SWS detector) VLT observation on W33A : HDO / H2O < 10-2(Dartois et al. 2003, A&A 399, 1009) Inconclusive mostly because high-mass protostars show a lower degree of deuteration than low-mass protostars ? Grain surface model predictions : HDO / H2O ~ 20 %

  10. Search for solid HDO in low-mass protostars grain mantles Recherche de HDO en phase solide sur les grains CH3 D:O2 -1:1 ice before and after UV irradiation (Dartois et al 2000) Observation of OD and OH stretch bands (in absorption) at 4.1 and 3 m with SpeX on IRTF (Mauna Kea) R = 1500

  11. Solid phase HDO/H2O (1) • 4 class I protostars • bright in NIR • high J-K extinction • D2CO/H2CO ~ 5% Parise et al. 2003, A&A 410, 897

  12. Solid phase HDO/H2O (2) HDO/H2O ≤ ~ 1% Does this exclude grain surface chemistry for deuteration ? Or has water a different fractionation than methanol ?

  13. Deuterated water in the gas phaseIRAS 16293 - Observations JCMT • JCMT and IRAM observations • ON-source & outflow • 5 lines detected on-source • no emission detected towards the outflow. IRAM

  14. Deuterated water in IRAS 16293 - Modelling • Model of envelope emission from • Ceccarelli, Hollenbach & Tielens 1996: • density structure : inside-out collapse (Shu scenario). • gas temperature computed self-consistently : cooling depends on CO, O and H2O abundance. • Adapted to HDO study : • collision coefficients from Green (1989) • use of the density and temperature profiles as well as water abundance derived by Ceccarelli et al. (2000). • HDO abundance : inner envelope : HDO/H2O = 3% outer envelope : HDO/H2O < 0.2 % Tev = 100 K xin xout R Parise et al. 2005, A&A 431, 547

  15. Questions raised by single dish observations • Deuterated methanol observations : • IRAS16293-2422CH2DOH/CH3OH = 37 % • CH3OD/CH3OH =1.8 % • CHD2OH/CH3OH = 7.4 % • CD3OH/CH3OH =1.0 % • Consistent with grain chemistry models, but : • these models require a high atomic D/H ratio in the gas phase, which can only occur when CO is heavily depleted. • these models predict HDO/H2O ~ 20 % ... ...HDO observation in grain mantles : • NGC1333 SVS12, SVS13, L1489 IRS, TMR1HDO/H2O ≤ 1 % • ...HDO observation in the gas phase : • IRAS16293-2422HDO/H2O = 3% in the inner enveloppe • HDO/H2O < 0.2 % in the outer enveloppe.

  16. Conclusions Modelling of the single-dish HDO emission points to a fractionation enhancement of water in the inner warm envelope (“hot corino”). Same for methanol ? Unfortunately such a modelling cannot be performed for CH2DOH because collision coefficients are not available. Moreover interferometric observations have shown that IRAS16293 is a binary with different chemical properties (PdBI, Bottinelli et al. 2004; SMA, Kuan et al. 2004, see poster by Huang et al). Accurate comparison to grain surface models can therefore only be done after observing the spatial distribution of deuterated methanol.

  17. Special thanks to the organizers of this conference. Thanks to my collaborators : Cecilia Ceccarelli - Emmanuel Caux Eric Herbst - A.G.G.M. Tielens Alain Castets - Bertrand Lefloch And all the WAGOS group Ted Simon - John Rayner Indra Mukhopadhyay Emmanuel Dartois - Laurent Loinard Peter Schilke - Karl Menten Operations at IRAM are funded by the CNRS (Centre National de la Recherche Scientifique, France), the MPG (Max Planck Gesellschaft, Germany), and the IGN (Instituto Geografico Nacional, Spain). The Infrared Telescope Facility is operated by the University of Hawaii under Cooperative Agreement no. NCC 5-538 with the National Aeronautics and Space Administration, Office of Space Science, Planetary Astronomy Program. The James Clerk Maxwell Telescope (JCMT) is operated by the Joint Astronomy Centre on behalf of the UK Particle Physics and Astronomy Research Council (PPARC), the National Research Council of Canada and the Netherlands Organisation for Pure Research. No animal was hurt during the preparation of this talk

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