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Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains

dc.contributor.authorGOICOECHEA, J.
dc.contributor.authorAGUADO, A.
dc.contributor.authorCUADRADO, S.
dc.contributor.authorRONCERO, O.
dc.contributor.authorPETY, J.
hal.structure.identifierLaboratoire d'Etude du Rayonnement et de la Matière en Astrophysique et Atmosphères = Laboratory for Studies of Radiation and Matter in Astrophysics and Atmospheres [LERMA]
dc.contributor.authorBRON, E.
dc.contributor.authorFUENTE, A.
dc.contributor.authorRIQUELME, D.
hal.structure.identifierInstitut de RadioAstronomie Millimétrique [IRAM]
hal.structure.identifierLaboratoire d'Astrophysique de Bordeaux [Pessac] [LAB]
dc.contributor.authorCHAPILLON, E.
dc.contributor.authorHERRERA, C.
dc.contributor.authorDURAN, C.
dc.date.issued2021-03
dc.identifier.issn0004-6361
dc.description.abstractEnHydride molecules lie at the base of interstellar chemistry, but the synthesis of sulfuretted hydrides is poorly understood and their abundances often crudely constrained. Motivated by new observations of the Orion Bar photodissociation region (PDR) – 1″ resolution ALMA images of SH + ; IRAM 30 m detections of bright H 2 32 S, H 2 34 S, and H 2 33 S lines; H 3 S + (upper limits); and SOFIA/GREAT observations of SH (upper limits) – we perform a systematic study of the chemistry of sulfur-bearing hydrides. We self-consistently determine their column densities using coupled excitation, radiative transfer as well as chemical formation and destruction models. We revise some of the key gas-phase reactions that lead to their chemical synthesis. This includes ab initio quantum calculations of the vibrational-state-dependent reactions SH + + H 2 ( v ) ⇄ H 2 S + + H and S + H 2 ( v ) ⇄ SH + H. We find that reactions of UV-pumped H 2 ( v ≥ 2) molecules with S + ions explain the presence of SH + in a high thermal-pressure gas component, P th ∕ k ≈ 10 8 cm −3 K, close to the H 2 dissociation front (at A V < 2 mag). These PDR layers are characterized by no or very little depletion of elemental sulfur from the gas. However, subsequent hydrogen abstraction reactions of SH + , H 2 S + , and S atoms with vibrationally excited H 2 , fail to form enough H 2 S + , H 3 S + , and SH to ultimately explain the observed H 2 S column density (~2.5 × 10 14 cm −2 , with an ortho-to-para ratio of 2.9 ± 0.3; consistent with the high-temperature statistical value). To overcome these bottlenecks, we build PDR models that include a simple network of grain surface reactions leading to the formation of solid H 2 S (s-H 2 S). The higher adsorption binding energies of S and SH suggested by recent studies imply that S atoms adsorb on grains (and form s-H 2 S) at warmer dust temperatures ( T d < 50 K) and closer to the UV-illuminated edges of molecular clouds. We show that everywhere s-H 2 S mantles form(ed), gas-phase H 2 S emission lines will be detectable. Photodesorption and, to a lesser extent, chemical desorption, produce roughly the same H 2 S column density (a few 10 14 cm −2 ) and abundance peak (a few 10 −8 ) nearly independently of n H and G 0 . This agrees with the observed H 2 S column density in the Orion Bar as well as at the edges of dark clouds without invoking substantial depletion of elemental sulfur abundances.
dc.language.isoen
dc.publisherEDP Sciences
dc.subject.enastrochemistry
dc.subject.enline: identification
dc.subject.enISM: clouds
dc.subject.enphoton-dominated region
dc.title.enBottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains
dc.typeArticle de revue
dc.identifier.doi10.1051/0004-6361/202039756
dc.subject.halPhysique [physics]
dc.subject.halPhysique [physics]/Astrophysique [astro-ph]
dc.identifier.arxiv2101.01012
bordeaux.journalAstronomy and Astrophysics - A&A
bordeaux.pageA10
bordeaux.volume647
bordeaux.peerReviewedoui
hal.identifierhal-03512037
hal.version1
hal.popularnon
hal.audienceInternationale
hal.origin.linkhttps://hal.archives-ouvertes.fr//hal-03512037v1
bordeaux.COinSctx_ver=Z39.88-2004&amp;rft_val_fmt=info:ofi/fmt:kev:mtx:journal&amp;rft.jtitle=Astronomy%20and%20Astrophysics%20-%20A&A&amp;rft.date=2021-03&amp;rft.volume=647&amp;rft.spage=A10&amp;rft.epage=A10&amp;rft.eissn=0004-6361&amp;rft.issn=0004-6361&amp;rft.au=GOICOECHEA,%20J.&amp;AGUADO,%20A.&amp;CUADRADO,%20S.&amp;RONCERO,%20O.&amp;PETY,%20J.&amp;rft.genre=article


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