Very cold dust associated with molecular gas

R. J. Laureijs - L. Haikala - M. Burgdorf - F.O. Clark - T. Liljeström - K. Mattila - T. Prusti

Abstract:

We report the detection of very cold dust in a small and isolated dust cloud. Observations at 60, 90, 135, and 200 microns show a constant surface brightness ratio I₁₃₅/I₂₀₀ and a flattening of I₉₀/I₂₀₀ towards the cloud centre. The dust temperature derived from the far-infrared colours can be confined to the range of 12-15 K assuming a Lambda^-2 dust emissivity. The 60 micron emission comes mainly from outer dust layers surrounding the core. We infer that a fraction of the power in the 90 micron band comes from the warmer grain component also causing I₆₀ . This fraction is approximately equal to the total power in the 60 micron band.

The cloud is also detected in ¹²CO, ¹³CO, ¹⁸CO, HCO⁺ and CS. The detections indicate that the dust emission is associated with a molecular cloud core where n(H₂) >= 3 10⁴ cm ^-3. Using the column density derived from the molecular observations we find an extinction cross section per H-atom of 4.1-9.5 cm²/H-atom.

Keywords:
interstellar medium: dust, extinction - interstellar medium: clouds - interstellar medium: molecules

Full text