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Astron. Astrophys. 315, L317-L320 (1996)

Very cold dust associated with molecular gas*

R.J. Laureijs1, L. Haikala2, M. Burgdorf1, F.O. Clark3, T. Liljeström2, K. Mattila2, and T. Prusti1

1 ISO Science Operations Centre, Astrophysics Division of ESA, Villafranca, Apdo 50727, E-28080 Madrid, Spain
2 Observatory, P.O. Box 14, FIN-00014 University of Helsinki, Finland
3 Geophysics Directorate - Phillips Laboratory, GPOB 29 Randolph Road, Hanscom Air Force Base, MA 01731, USA

Received 15 July 1996 / Accepted 3 September 1996

Abstract. We report the detection of very cold dust in a small and isolated dust cloud. Observations at 60, 90, 135, and 200  img2.gifm show a constant surface brightness ratio img3.gif and a flattening of img4.gif 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 img5.gif dust emissivity. The 60 img2.gifm emission comes mainly from outer dust layers surrounding the core. We infer that a fraction of the power in the 90 img2.gifm band comes from the warmer grain component also causing img6.gif . This fraction is approximately equal to the total power in the 60 img2.gifm band.

The cloud is also detected in 12CO, 13CO, 18CO, img10.gif and CS. The detections indicate that the dust emission is associated with a molecular cloud core where img11.gif . Using the column density derived from the molecular observations we find an extinction cross section per H-atom of 4.1-9.5 img12.gif .

Key words: interstellar medium: dust, extinction - interstellar medium: clouds - interstellar medium: molecules

* Based on observations made with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA

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