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

We present submillimeter observations between 200 and 600 mu m obtained wit h the French balloon-borne instrument SPM/PRONAOS toward the high latitude cirrus cloud MCLD 123.5 + 24.9 in the Polaris Flare. These sensitive measur ements allow for the first time to study in details the submillimeter dust emission of cirruses at the are minute scale. The extended cirrus as well a s a compact condensation already identified as an NH3 core are well detecte d in the four photometric channels of the instrument. We also present preli minary results of observations of the same region at 150 mu m obtained with PHOT on board the ISO satellite. The data are consistent with emission fro m cold dust at 13 K with a steep dust emissivity index of beta = 2.2 for th e extended cirrus. This value is consistent with, although slightly higher than the one used to describe the FIRAS data at high latitude (beta = 2.0). It is compatible with recent laboratory measurements for amorphous silicat es at low temperature which indicate beta > 2.0. We show that the low value of the dust temperature for the extended cirrus cannot be explained by ext inction from the surrounding medium and/or internal extinction in the cloud . We propose that the optical properties of dust in regions such as the Polar is cirrus, which exhibit large 100 mu m excess over their 60 mu m emission, are modified compared to those prevailing in the general diffuse interstel lar medium in the Galaxy. This change in the optical properties of large gr ains is likely connected to the coagulation of smaller dust particles on la rge size grains.