Universität zu Köln

Parikka, A., Habart, E., Bernard-Salas, J., Goicoechea, J. R., Abergel, A., Pilleri, P., Dartois, E., Joblin, C., Gerin, M. and Godard, B. (2017). Spatial distribution of far-infrared rotationally excited CH+ and OH emission lines in the Orion Bar photodissociation region. Astron. Astrophys., 599. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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Abstract

Context. The methylidyne cation (CH+) and hydroxyl (OH) are key molecules in the warm interstellar chemistry, but their formation and excitation mechanisms are not well understood. Their abundance and excitation are predicted to be enhanced by the presence of vibrationally excited H-2 or hot gas (similar to 500-1000 K) in photodissociation regions (PDRs) with high incident far-ultraviolet (FUV) radiation field. The excitation may also originate in dense gas (>10(5) cm(-3)) followed by nonreactive collisions with H-2, H, and electrons. Previous observations of the Orion Bar suggest that the rotationally excited CH+ and OH correlate with the excited CO, which is a tracer of dense and warm gas, and that formation pumping contributes to CH+ excitation. Aims. Our goal is to examine the spatial distribution of the rotationally excited CH+ and OH emission lines in the Orion Bar to establish their physical origin and main formation and excitation mechanisms. Methods. We present spatially sampled maps of the CH+ J = 3-2 transition at 119.8 mu m and the OH Lambda doublet at 84 mu m in the Orion Bar over an area of 110 '' Chi 110 '' with Herschel/PACS. We compare the spatial distribution of these molecules with those of their chemical precursors, C-broken vertical bar, O and H-2, and tracers of warm and dense gas (high-J CO). We assess the spatial variation of the CH+ J = 2-1 velocity-resolved line profile at 1669 GHz with Herschel/HIFI spectrometer observations. Results. The OH and especially CH+ lines correlate well with the high-J CO emission and delineate the warm and dense molecular region at the edge of the Bar. While notably similar, the differences in the CH+ and OH morphologies indicate that CH+ formation and excitation are strongly related to the observed vibrationally excited H-2. This, together with the observed broad CH+ line widths, indicates that formation pumping contributes to the excitation of this reactive molecular ion. Interestingly, the peak of the rotationally excited OH 84 mu m emission coincides with a bright young object, proplyd 244-440, which shows that OH can be an excellent tracer of UV-irradiated dense gas. Conclusions. The spatial distribution of CH+ and OH revealed in our maps is consistent with previous modeling studies. Both formation pumping and nonreactive collisions in a UV-irradiated dense gas are important CH+ J = 3-2 excitation processes. The excitation of the OH Lambda doublet at 84 mu m is mainly sensitive to the temperature and density.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Parikka, A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Habart, E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Bernard-Salas, J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Goicoechea, J. R. UNSPECIFIED UNSPECIFIED UNSPECIFIED Abergel, A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Pilleri, P. UNSPECIFIED UNSPECIFIED UNSPECIFIED Dartois, E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Joblin, C. UNSPECIFIED UNSPECIFIED UNSPECIFIED Gerin, M. UNSPECIFIED UNSPECIFIED UNSPECIFIED Godard, B. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-237969
DOI:	10.1051/0004-6361/201629445
Journal or Publication Title:	Astron. Astrophys.
Volume:	599
Date:	2017
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 STAR-FORMING REGIONS; SUB-DOPPLER MEASUREMENTS; PHOTON-DOMINATED REGION; INTERSTELLAR-MEDIUM; COLLISIONAL EXCITATION; PHYSICAL CONDITIONS; ABSORPTION-LINES; MOLECULAR CLOUD; LOW-TEMPERATURE; CROSS-SECTIONS Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/23796