The different structures of the two classes of starless cores

We describe a model for the thermal and dynamical equilibrium of starless cores that includes the radiative transfer of gas and dust and simple CO chemistry. The model shows that the structure and behavior of the cores is significantly different depending on whether the central density is either above or below about 105 cm 3. This density is significant as the critical density for gas cooling by gas- dust collisions and as the critical density for dynamical stability, given the typical properties of the starless cores. Starless cores thus divide into two classes that we refer to as thermally supercritical and thermally subcritical. This two- class distinction allows an improved interpretation of the different observational data of starless cores within a single model.