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

We present submillimeter observations between 200 and 600μm obtained with the French balloon-borne instrument SPM/PRONAOS toward the high latitude cirrus cloud MCLD123.5 + 24.9 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 150μm 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 β = 2.2 for the extended cirrus. This value is consistent with, although slightly higher than the one used to describe the FIRAS data at high latitude (β = 2.0). It is compatible with recent laboratory measurements for amorphous silicates at low temperature which indicate β > 2.0. 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 100μm excess over their 60μm 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.