Three-dimensional distribution of hydrogen fluoride gas toward NGC 6334 I and I(N)

Abstract

Aims. We investigate the spatial distribution of a collection of absorbing gas clouds, some associated with the dense, massive star-forming core NGC6334 I, and others with diffuse foreground clouds. For the former category, we aim to study the dynamical properties of the clouds in order to assess their potential to feed the accreting protostellar cores. Methods. We use spectral imaging from the Herschel SPIRE iFTS to construct a map of HF absorption at 243 micron in a 6x3.5 arcmin region surrounding NGC6334 I and I(N). Results. The combination of new, spatially fully sampled, but spectrally unresolved mapping with a previous, single-pointing, spectrally resolved HF signature yields a 3D picture of absorbing gas clouds in the direction of NGC6334. Toward core I, the HF equivalent width matches that of the spectrally resolved observation. The distribution of HF absorption is consistent with three of the seven components being associated with this dense star-forming envelope. For two of the remaining four components, our data suggest that these clouds are spatially associated with the larger scale filamentary star-forming complex. Our data also implies a lack of gas phase HF in the envelope of core I(N). Using a simple description of adsorption onto and desorption from dust grain surfaces, we show that the overall lower temperature of the envelope of source I(N) is consistent with freeze-out of HF, while it remains in the gas phase in source I. Conclusions. We use the HF molecule as a tracer of column density in diffuse gas (n(H) ~ 10^2 - 10^3 cm^-3), and find that it may uniquely trace a relatively low density portion of the gas reservoir available for star formation that otherwise escapes detection. At higher densities prevailing in protostellar envelopes (>10^4 cm^-3), we find evidence of HF depletion from the gas phase under sufficiently cold conditions.