Very cold dust associated with molecular gas.

Abstract

We report the detection of very cold dust in a small and isolated dust cloud. Observations at 60, 90, 135, and 200μm show a constant surface brightness ratio I_135_/I_200_ and a flattening of I_90_/I_200_ towards the cloud centre. The dust temperature derived from the far-infrared colours can be confined to the range of 12-15K assuming a λ^-2^ dust emissivity. The 60μm emission comes mainly from outer dust layers surrounding the core. We infer that a fraction of the power in the 90μm band comes from the warmer grain component also causing I_60_. This fraction is approximately equal to the total power in the 60μm band. The cloud is also detected in ^12^CO, ^13^CO, CO^18^, HCO^+^ and CS. The detections indicate that the dust emission is associated with a molecular cloud core where n(H_2_) >=3x10^4^cm^-3^. Using the column density derived from the molecular observations we find an extinction cross section per H-atom of 4.1-9.5cm^2^/H-atom.