Universität zu Köln

van der Tak, F. F. S., Nagy, Z., Ossenkopf, V., Makai, Z., Black, J. H., Faure, A., Gerin, M. and Bergin, E. A. (2013). Spatially extended OH+ emission from the Orion Bar and Ridge. Astron. Astrophys., 560. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

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 similar to 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 approximate to 4 kms(-1) suggests an origin of the OH+ emission close to the PDR surface, at a depth of A(V) similar to 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 similar to 1.0 x 10(14) cm(-2) is similar to that in previous absorption line studies, while our limits on the ratios of OH+/H2O+ (greater than or similar to 40) and OH+/H3O+ (greater than or similar to 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 H-2, 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.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code van der Tak, F. F. S. UNSPECIFIED UNSPECIFIED UNSPECIFIED Nagy, Z. UNSPECIFIED UNSPECIFIED UNSPECIFIED Ossenkopf, V. UNSPECIFIED UNSPECIFIED UNSPECIFIED Makai, Z. UNSPECIFIED UNSPECIFIED UNSPECIFIED Black, J. H. UNSPECIFIED UNSPECIFIED UNSPECIFIED Faure, A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Gerin, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Bergin, E. A. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-470546
DOI:	10.1051/0004-6361/201322164
Journal or Publication Title:	Astron. Astrophys.
Volume:	560
Date:	2013
Publisher:	EDP SCIENCES S A
Place of Publication:	LES ULIS CEDEX A
ISSN:	1432-0746
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language RAY IONIZATION RATE; STAR-FORMING REGIONS; MOLECULAR CLOUDS; INTERSTELLAR OH+; HERSCHEL-SPIRE; ROTATIONAL-EXCITATION; ELECTRON-DENSITIES; ATOMIC-HYDROGEN; DARK CLOUDS; ARP 220 Multiple languages Astronomy & Astrophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/47054