The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+ The effect of high electron density and vibrationally excited H-2 in a warm PDR surface
Journal article, 2013

Context. The abundances of interstellar CH+ and SH+ are not well understood as their most likely formation channels are highly endothermic. Several mechanisms have been proposed to overcome the high activation barriers, including shocks, turbulence, and H-2 vibrational excitation. Aims. Using data from the Herschel Space Observatory, we studied the formation of ions, in particular CH+ and SH+ in a typical high UV-illumination warm and dense photon-dominated region (PDR), the Orion Bar. Methods. The HIFI instrument on board Herschel provides velocity-resolved line profiles of CH+ 1-0 and 2-1 and three hyperfine transitions of SH+ 1(2)-0(1). The PACS instrument provides information on the excitation and spatial distribution of CH+ by extending the observed CH+ transitions up to J = 6-5. We compared the observed line intensities to the predictions of radiative transfer and PDR codes. Results. All CH+, SH+, and CF+ lines analyzed in this paper are seen in emission. The widths of the CH+ 2-1 and 1-0 transitions are of similar to 5 kms(-1), significantly broader than the typical width of dense gas tracers in the Orion Bar (similar to 2-3 km s(-1)) and are comparable to the width of species that trace the interclump medium such as C+ and HF. The detected SH+ transitions are narrower compared to CH+ and have line widths of similar to 3 kms(-1), indicating that SH+ emission mainly originates in denser condensations. Non-LTE radiative transfer models show that electron collisions affect the excitation of CH+ and SH+ and that reactive collisions need to be taken into account to calculate the excitation of CH+. Comparison to PDR models shows that CH+ and SH+ are tracers of the warm surface region (A(V) < 1.5) of the PDR with temperatures between 500 and 1000 K. We have also detected the 5-4 transition of CF+ at a width of similar to 1.9 kms(-1), consistent with the width of dense gas tracers. The intensity of the CF+ 5-4 transition is consistent with previous observations of lower-J transitions toward the Orion Bar. Conclusions. An analytic approximation and a numerical comparison to PDR models indicate that the internal vibrational energy of H-2 can explain the formation of CH+ for typical physical conditions in the Orion Bar near the ionization front. The formation of SH+ is also likely to be explained by H-2 vibrational excitation. The abundance ratios of CH+ and SH+ trace the destruction paths of these ions, and indirectly, the ratios of H, H-2, and electron abundances as a function of depth into the cloud.

ISM: individual objects: Orion Bar

ISM: molecules


Z. Nagy

University of Groningen

Netherlands Institute for Space Research (SRON)

F. F. S. van der Tak

University of Groningen

Netherlands Institute for Space Research (SRON)

V. Ossenkopf

University of Cologne

M. Gerin

Ecole Normale Superieure (ENS)

F. Le Petit

Paris Diderot University

J. Le Bourlot

Paris Diderot University

John H Black

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

J. R. Goicoechea

Centro de Astrobiologia (CAB)

C. Joblin

University of Toulouse

Centre national de la recherche scientifique (CNRS)

M. Rollig

University of Cologne

E. A. Bergin

University of Michigan

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 550 A96- A96

Subject Categories

Astronomy, Astrophysics and Cosmology


Basic sciences


Onsala Space Observatory



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