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Astron. Astrophys. 347, 640-649 (1999)

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PRONAOS observations of MCLD 123.5 + 24.9: cold dust in the Polaris cirrus cloud

J.P. Bernard 1, A. Abergel 1, I. Ristorcelli 2, F. Pajot 1, J.P. Torre 3, F. Boulanger 1, M. Giard 2, G. Lagache 1, G. Serra 2, J.M. Lamarre 1, J.L. Puget 1, F. Lepeintre 1 and L. Cambrésy 4

1 IAS, bâtiment 121, Campus d'Orsay, F-91405 Orsay Cedex, France
2 CESR, 9 Avenue du Colonel Roche, B.P. 4346, F-31029 Toulouse, France
3 SA, UPR-CNRS, Verrières-le Buisson, France
4 DESPA, Observatoire de Paris, 5 Place Jules Janssen, F-92195 Meudon Cedex, France

Received 7 January 1999 / Accepted 18 March 1999


We present submillimeter observations between 200 and [FORMULA] obtained with the French balloon-borne instrument SPM/PRONAOS toward the high latitude cirrus cloud [FORMULA] in the Polaris Flare. These sensitive measurements allow for the first time to study in details the submillimeter dust emission of cirruses at the arc minute scale. The extended cirrus as well as a compact condensation already identified as an NH3 core are well detected in the four photometric channels of the instrument. We also present preliminary results of observations of the same region at [FORMULA] obtained with PHOT on board the ISO satellite. The data are consistent with emission from cold dust at 13 K with a steep dust emissivity index of [FORMULA] for the extended cirrus. This value is consistent with, although slightly higher than the one used to describe the FIRAS data at high latitude ([FORMULA]). It is compatible with recent laboratory measurements for amorphous silicates at low temperature which indicate [FORMULA]. We show that the low value of the dust temperature for the extended cirrus cannot be explained by extinction from the surrounding medium and/or internal extinction in the cloud.

We propose that the optical properties of dust in regions such as the Polaris cirrus, which exhibit large [FORMULA] excess over their [FORMULA] emission, are modified compared to those prevailing in the general diffuse interstellar medium in the Galaxy. This change in the optical properties of large grains is likely connected to the coagulation of smaller dust particles on large size grains.

Key words: ISM: clouds – ISM: dust, extinction – ISM: individual objects: MCLD 123.5+24.9 – infrared: ISM: continuum – submillimeter

Send offprint requests to: J.P. Bernard (bernard@ias.fr)

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999