Spatially extended OH+ emission from the Orion Bar and Ridge

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van der Tak, F. F. S.Rijksuniversiteit Groningen,University of Groningen,Netherlands Institute for Space Research (SRON) (författare)

Spatially extended OH+ emission from the Orion Bar and Ridge

Artikel/kapitelEngelska2013

Förlag, utgivningsår, omfång ...

2013-12-11
EDP Sciences,2013
electronicrdacarrier

Nummerbeteckningar

LIBRIS-ID:oai:research.chalmers.se:e7555355-ad23-4f2e-bafa-b81500d4522a
https://research.chalmers.se/publication/191285URI
https://doi.org/10.1051/0004-6361/201322164DOI

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Språk:engelska
Sammanfattning på:engelska

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Ämneskategori:art swepub-publicationtype
Ämneskategori:ref swepub-contenttype

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Context. The reactive HnO+ ions (OH+, H2O+ and H3O+) are widespread in the interstellar medium and act as precursors to the H2O molecule. While HnO+ absorption is seen on many Galactic lines of sight, active galactic nuclei often show the lines in emission. Aims: This paper shows the first example of a Galactic source of HnO+ line emission: the Orion Bar, a bright nearby photon-dominated region (PDR). Methods: We present line profiles and maps of OH+ line emission toward the Orion Bar, and upper limits to H2O+ and H3O+ lines. We analyze these HIFI data with non-local thermodynamic equilibrium radiative transfer and PDR chemical models, using newly calculated inelastic collision data for the e-OH+ system. Results: Line emission is detected over ~1' (0.12 pc), tracing the Bar itself as well as a perpendicular feature identified as the southern tip of the Orion Ridge, which borders the Orion Nebula on its western side. The line width of ≈ 4 km s-1 suggests an origin of the OH+ emission close to the PDR surface, at a depth of AV ~ 0.3-0.5 into the cloud where most hydrogen is in atomic form. Steady-state collisional and radiative excitation models for OH+ require unrealistically high column densities to match the observed line intensity, indicating that the formation of OH+ in the Bar is rapid enough to influence its excitation. Our best-fit OH+ column density of ~ 1.0 × 1014 cm-2 is similar to that in previous absorption line studies, while our limits on the ratios of OH+/H2O+ (≳ 40) and OH+/H3O+ (≳ 15) are somewhat higher than seen before. Conclusions: The column density of OH+ is consistent with estimates from a thermo-chemical model for parameters applicable to the Orion Bar, given the current uncertainties in the local gas pressure and the spectral shape of the ionizing radiation field. The unusually high OH+/H2O+ and OH+/H3O+ ratios are probably due to the high UV radiation field and electron density in this object. In the Bar, photodissociation and electron recombination are more effective destroyers of OH+ than the reaction with H2, which limits the production of H2O+. The appearance of the OH+ lines in emission is the result of the high density of electrons and H atoms in the Orion Bar, since for these species, inelastic collisions with OH+ are faster than reactive ones. In addition, chemical pumping, far-infrared pumping by local dust, and near-UV pumping by Trapezium starlight contribute to the OH+ excitation. Similar conditions may apply to extragalactic nuclei where HnO+ lines are seen in emission.

Ämnesord och genrebeteckningar

NATURVETENSKAP Fysik Astronomi, astrofysik och kosmologi hsv//swe
NATURAL SCIENCES Physical Sciences Astronomy, Astrophysics and Cosmology hsv//eng
molecules
interstellar matter
astrochemistry

Biuppslag (personer, institutioner, konferenser, titlar ...)

Nagy, ZsofiaNetherlands Institute for Space Research (SRON),Rijksuniversiteit Groningen,University of Groningen (författare)
Ossenkopf, V.Universität zu Köln,University of Cologne (författare)
Makai, Z.Universität zu Köln,University of Cologne (författare)
Black, John H,1949Chalmers tekniska högskola,Chalmers University of Technology(Swepub:cth)jblack (författare)
Faure, A.Université Grenoble Alpes,Grenoble Alpes University (författare)
Gerin, M.LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres, France (författare)
Bergin, E. A.University of Michigan (författare)
Rijksuniversiteit GroningenNetherlands Institute for Space Research (SRON) (creator_code:org_t)

Sammanhörande titlar

Ingår i:Astronomy and Astrophysics: EDP Sciences560, s. A95 (pp. 1-10)0004-63611432-0746

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