Contrasting [CII], methylidynium ion CH^+, and methanol physical and chemical tracers around Orion KL.

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Scientific paper

Spectral maps with high velocity resolution and S/N ratios have been taken as part of the HEXOS Key Program with Herschel/HIFI of the inner region of the Orion KL nebula (typically 2' x 2') around the deeply embedded young massive star IRc2 at the [CII] 1900 GHz and CH+ J=1-0 835 and 2-1 1669 GHz. The CH+ 1-0 map also contains E symmetry K=5-4 Q branch emission lines. These maps provide excellent contrasts of the velocity and temperature/density structure, and isotopic structure of the cooling regions which have accommodated molecular cloud contraction and subsequent star formation. We present the distribution of the intense [CII] and CH+ lines from the turbulent hot and dense core and the surrounding relatively cool material, exhibiting a wide range of velocity profiles and 12C/13C isotopologue ratios. This allows a test of the previously deduced agreement of the isotope ratio with PDR models which imply that chemical fractionation in this highly illuminated region is unimportant, but we further consider optical depth effects which may attenuate the apparent abundance ratios. The CH+ J=1-0 transition is observed in emission over the entire map while the J=2-1 is primarily in absorption. The excitation of CH+ is problematic due to its rapid reactions with H2 and electrons. The line width is comparable to other extended molecules. Possible mechanisms for excitation will be discussed in light of the observational data, in relation to the photodissociation surface illuminated by theta1 Ori C to the south of IRc2.

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