Parts of the 100 micron emission of near-by clouds have no or
 little counterpart at 60 micron. This cold component display
 in several clouds a spectacular network of small and
 filament-like cold structures. At the edge of the cold
 structures, the 100 micron gradient is often not resolved by
 IRAS. In star forming regi ons, most of
 young protostellar objects are included inside these cold
 emission regions.
 The goal of this proposal is to understand the physical
 processes giving rise to these cold emission (increase in
 size of very small grains by mantle formation or mutual
 coagulation of grains?) and the connection between
 the evolution of the dust and the condensation of matter.

 We propose to map several cold structures on large  scale
 (30' or more) at 90 and 200 micron with PHT-C. The spatial
 resolution should allow to resolve the transition regions,
 and observations at wavelength longer than IRAS may reveal
 color changes within the cold regions. Selected regions
 have no identified star formation and limited extinctions.