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In contrast to extensively studied dense star-forming cores, little is knownabout diffuse gas surrounding star-forming regions. We study molecular gas inthe high-mass star-forming region NGC6334I, which contains diffuse, quiescentcomponents that are inconspicuous in widely used molecular tracers such as CO.We present Herschel/HIFI observations of CH toward NGC6334I observed as part ofthe CHESS key program. HIFI resolves the hyperfine components of its J=3/2-1/2transition, observed in both emission and absorption. The CH emission appearsclose to the systemic velocity of NGC6334I, while its measured linewidth of 3km/s is smaller than previously observed in dense gas tracers such as NH3 andSiO. The CH abundance in the hot core is 7 10^-11, two to three orders ofmagnitude lower than in diffuse clouds. While other studies find distinctoutflows in, e.g., CO and H2O toward NGC6334I, we do not detect outflowsignatures in CH. To explain the absorption signatures, at least two absorbingcomponents are needed at -3.0 and +6.5 km/s with N(CH)=7 10^13 and 3 10^13cm^-2. Two additional absorbing clouds are found at +8.0 and 0.0 km/s, bothwith N(CH)=2 10^13 cm^-2. Turbulent linewidths for the four absorptioncomponents vary between 1.5 and 5.0 km/s in FWHM. We constrain physicalproperties of our CH clouds by matching our CH absorbers with other absorptionsignatures. In the hot core, molecules such as H2O and CO trace gas that isheated and dynamically influenced by outflow activity, whereas CH traces morequiescent material. The four CH absorbers have column densities and turbulentproperties consistent with diffuse clouds: two are located near NGC6334, andtwo are unrelated foreground clouds. Local density and dynamical effectsinfluence the chemical composition of physical components of NGC6334, causingsome components to be seen in CH but not in other tracers, and vice versa.

Herschel/HIFI observations of spectrally resolved methylidyne signatures toward the high-mass star-forming core NGC6334I