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

Abstract

In contrast to extensively studied dense star-forming cores, little is known about diffuse gas surrounding star-forming regions. We study molecular gas in the 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 CH toward NGC6334I observed as part of the CHESS key program. HIFI resolves the hyperfine components of its J=3/2-1/2 transition, observed in both emission and absorption. The CH emission appears close to the systemic velocity of NGC6334I, while its measured linewidth of 3 km/s is smaller than previously observed in dense gas tracers such as NH3 and SiO. The CH abundance in the hot core is 7 10^-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 outflow signatures in CH. To explain the absorption signatures, at least two absorbing components are needed at -3.0 and +6.5 km/s with N(CH)=7 10^13 and 3 10^13 cm^-2. Two additional absorbing clouds are found at +8.0 and 0.0 km/s, both with N(CH)=2 10^13 cm^-2. Turbulent linewidths for the four absorption components vary between 1.5 and 5.0 km/s in FWHM. We constrain physical properties of our CH clouds by matching our CH absorbers with other absorption signatures. In the hot core, molecules such as H2O and CO trace gas that is heated and dynamically influenced by outflow activity, whereas CH traces more quiescent material. The four CH absorbers have column densities and turbulent properties consistent with diffuse clouds: two are located near NGC6334, and two are unrelated foreground clouds. Local density and dynamical effects influence the chemical composition of physical components of NGC6334, causing some components to be seen in CH but not in other tracers, and vice versa.

arXiv commentAccepted by A&A Letters; 5 pages, 1 figure; v2: minor textual and typographical changes

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• Herschel/HIFI discovery of interstellar chloronium (H${}_2$Cl${}^{+}$)
• Water in star-forming regions with Herschel (WISH). IV. A survey of low-J H2O line profiles toward high-mass protostars
• Molecular line survey of the high-mass star-forming region NGC 6334I with Herschel/HIFI and the SMA
• The Abundance, Ortho/Para Ratio, and Deuteration of Water in the High-Mass Star Forming Region NGC 6334 I
• WISH VI. Constraints on UV and X-ray irradiation from a survey of hydrides in low- to high-mass YSOs
• Herschel observations of extra-ordinary sources: Detecting spiral arm clouds by CH absorption lines
• The distribution of water in the high-mass star-forming region NGC 6334I
• Ionization toward the high-mass star-forming region NGC 6334 I
• Measurement of the lowest millimetre-wave transition frequency of the CH radical
• Three-dimensional distribution of hydrogen fluoride gas toward NGC6334 I and I(N)