Relevance of the H_2 + O reaction pathway for the surface formation of interstellar water. Combined experimental and modeling study
Abstract
The formation of interstellar water is commonly accepted to occur on the surfaces of icy dust grains in dark molecular clouds at low temperatures (10–20 K), involving hydrogenation reactions of oxygen allotropes. As a result of the large abundances of molecular hydrogen and atomic oxygen in these regions, the reaction H_2 + O has been proposed to contribute significantly to the formation of water as well. However, gas-phase experiments and calculations, as well as solid-phase experimental work contradict this hypothesis. Here, we use precisely executed temperature-programmed desorption (TPD) experiments in an ultra-high vacuum setup combined with kinetic Monte Carlo simulations to establish an upper limit of the water production starting from H_2 and O. These reactants were brought together in a matrix of CO_2 in a series of (control) experiments at different temperatures and with different isotopological compositions. The water detected with the quadrupole mass spectrometer upon TPD was found to originate mainly from contamination in the chamber itself. However, if water is produced in small quantities on the surface through H_2 + O, this can only be explained by a combined classical and tunneled reaction mechanism. An absolutely conservative upper limit for the reaction rate was derived with a microscopic kinetic Monte Carlo model that converts the upper limit into the highest possible reaction rate. Incorporating this rate into simulation runs for astrochemically relevant parameters shows that the upper limit to the contribution of the reaction H_2 + O in OH, and hence water formation, is 11% in dense interstellar clouds. Our combined experimental and theoretical results indicate, however, that this contribution is most likely much lower.
Additional Information
© 2014 ESO. Article published by EDP Sciences. Received 22 May 2014. Accepted 9 September 2014. Published online 16 October 2014. H.M.C. is grateful for support from the VIDI research program 700.10.427, which is financed by The Netherlands Organization for Scientific Research (NWO) and from the European Research Council (ERC-2010-StG, Grant Agreement no. 259510-KISMOL). T.L. is supported by the Dutch Astrochemistry Network financed by The Netherlands Organization for Scientific Research (NWO). Support for S.I. from the Niels Stensen Fellowship and the Marie Curie Fellowship (FP7-PEOPLE-2011-IOF-300957) is gratefully acknowledged. The SLA group has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement n.238258, the Netherlands Research School for Astronomy (NOVA) and from the Netherlands Organization for Scientific Research (NWO) through a VICI grant.Attached Files
Published - aa24252-14.pdf
Submitted - 1409.3055v1.pdf
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Additional details
- Alternative title
- On the relevance of the H2 + O reaction pathway for the surface formation of interstellar water - A combined experimental and modeling study
- Alternative title
- Relevance of the H2 + O reaction pathway for the surface formation of interstellar water
- Eprint ID
- 52446
- Resolver ID
- CaltechAUTHORS:20141205-152024638
- 700.10.427
- Netherlands Organization for Scientific Research (NWO) VIDI research program
- 259510-KISMOL
- European Research Council ERC-2010-StG
- Dutch Astrochemistry Network
- Niels Stensen Fellowship
- FP7-PEOPLE-2011-IOF-300957
- Marie Curie Fellowship
- 238258
- European Community Seventh Framework Programme (FP7/2007-2013)
- Netherlands Research School for Astronomy (NOVA)
- Netherlands Organization for Scientific Research (NWO) VICI grant
- Created
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2014-12-05Created from EPrint's datestamp field
- Updated
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2021-11-10Created from EPrint's last_modified field