The IRAM key-project: small-scale structure of pre-star forming regions

We present a subset of the results of the first IRAM key-project. This project is devoted to the analysis of the processes which drive the dissipation of the supersonic non-thermal support of molecular clouds, known to be at the origin of their gravitational stability. One of the outcomes of this energy dissipation is expected to be the formation of thermally supported structures, such as the dense cores, observed in molecular clouds with an internal velocity dispersion close to thermal, and often associated with extremely young stellar sources. We have mapped several fields of nearby molecular clouds which all contain a starless dense core, in five rotational lines of CO isotopes, at high angular resolution. The data set, because of its size and quality provides several new results such as a brightness temperature ratio for the one and two rotational transition close to uniform and the presence of unresolved filamentary structure. In a preliminary interpretation, we use the different properties of the spectral properties of the fields to derive an upper limit of a few 100 AU for the size of the line forming regions.