2010A&A...521L..43V

In contrast to the more extensively studied dense star-forming cores, little is known about diffuse gas surrounding star-forming regions. We study the molecular gas in the Galactic high-mass star-forming region NGC6334I, which contains diffuse, quiescent components that are inconspicuous in widely used molecular tracers such as CO. We present Herschel/HIFI observations of methylidyne (CH) toward NGC6334I observed as part of the ``Chemical HErschel Survey of Star forming regions'' (CHESS) key program. HIFI resolves each of the six hyperfine components of the lowest rotational transition (J=(3)/(2)-(1)/(2)) of CH, observed in both emission and absorption. The CH emission features appear close to the systemic velocity of NGC6334I, while its measured FWHM linewidth of 3km/s is smaller than previously observed in dense gas tracers such as NH₃ and SiO. The CH abundance in the hot core is ∼7x10^–11, two to three orders of magnitude lower than in diffuse clouds. While other studies find distinct outflows in, e.g., CO and H₂O toward NGC6334I, we do not detect any outflow signatures in CH. At least two redshifted components of cold absorbing material must be present at -3.0 and +6.5km/s to explain the absorption signatures. We derive a CH column density (N_CH) of 7x10¹³ and 3x10¹³cm^–2 for these two absorbing clouds. We find evidence of two additional absorbing clouds at +8.0 and 0.0km/s, both with N_CH≃ 2x10¹³cm^–2. Turbulent linewidths for the four absorption components vary between 1.5 and 5.0km/s in FWHM. We constrain the physical properties and locations of the clouds by matching our CH absorbers with the absorption signatures seen in other molecular tracers. In the hot core, molecules such as H₂O and CO trace gas that is heated and dynamically influenced by outflow activity, whereas the CH molecule traces more quiescent material. The four CH absorbing clouds have column densities and turbulent properties that are consistent with those of diffuse clouds: two are located in the direct surroundings of NGC6334, and two are unrelated foreground clouds. Local density and dynamical effects influence the chemical composition of the physical components of NGC6334, which causes some components to be seen in CH but not in other tracers, and vice versa.