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dc.contributor.authorThi, W. F.
dc.contributor.authorHocuk, S.
dc.contributor.authorKamp, I.
dc.contributor.authorWoitke, P.
dc.contributor.authorRab, Ch
dc.contributor.authorCazaux, S.
dc.contributor.authorCaselli, P.
dc.identifier.citationThi , W F , Hocuk , S , Kamp , I , Woitke , P , Rab , C , Cazaux , S & Caselli , P 2018 , ' Warm dust surface chemistry. H 2 and HD formation ' , Astronomy & Astrophysics .en
dc.identifier.otherPURE: 258237686
dc.identifier.otherPURE UUID: f71d1350-2982-44e1-b977-48985c7e3620
dc.descriptionFunding: EU FP7- 2011 under Grant Agreement nr. 284405 (IK, WFT, CR, and PW).en
dc.description.abstractMolecular hydrogen (H2) is the main constituent of the gas in the planet-forming disks that surround many PMS stars. H2 can be incorporated in the atmosphere of the giant planets. HD has been detected in a few disks and can be considered the most reliable tracer of H2. We wish to form H2 and HD efficiently for the varied conditions encountered in protoplanetary disks: the densities vary from 104 to 1016 cm-3; the dust temperatures range from 5 to 1500 K, the gas temperatures go from 5 to a few 1000 Kelvin, and the ultraviolet field can be 107 stronger than the standard interstellar field. We implemented a comprehensive model of H2 and HD formation on cold and warm grain surfaces and via hydrogenated PAHs in the physico-chemical code ProDiMo (PROtoplanetary DIsk MOdel). The H2 and HD formation can proceed via the Langmuir-Hinshelwood and Eley-Ridel mechanisms for physisorbed or chemisorbed H (D) atoms. H2 and HD also form by H (D) abstraction from hydrogenated neutral and ionised PAHs and via gas phase reactions. H2 and HD are formed efficiently on dust grain surfaces from 10 to 700 K. All the deuterium is converted into HD in UV shielded regions as soon as H2 is formed by gas-phase D abstraction reactions. The detailed model compares well with standard analytical prescriptions for H2 (HD) formation. At low temperatures, H2 is formed from the encounter of two physisorbed atoms. HD molecules form on the grain surfaces and in the gas-phase. At temperatures greater than 20 K, the meeting between a weakly bound H- (or D-) atom or a gas-phase H (D) atom and a chemisorbed atom is the most efficient H2 formation route. H2 formation through hydrogenated PAHs alone is efficient above 80 K. The contribution of hydrogenated PAHs to the overall H2 and HD formation is relatively low if chemisorption on silicate is taken into account and if a small hydrogen abstraction cross-section is used. The H2 and HD warm grain surface network is a first step in the construction of a network for high-temperature surface reactions.
dc.relation.ispartofAstronomy & Astrophysicsen
dc.rights© 2019, ESO. This work has been made available online in accordance with the publisher's policies. This is the author created accepted version manuscript following peer review and as such may differ slightly from the final published version. The final published version of this work is available at
dc.subjectMolecular processesen
dc.subjectMethods: numericalen
dc.subjectQB Astronomyen
dc.subjectQC Physicsen
dc.subjectQD Chemistryen
dc.titleWarm dust surface chemistry. H2 and HD formationen
dc.typeJournal articleen
dc.contributor.institutionUniversity of St Andrews.School of Physics and Astronomyen
dc.contributor.institutionUniversity of St Andrews.St Andrews Centre for Exoplanet Scienceen
dc.description.statusPeer revieweden

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