STRUCTURE AND PHYSICS OF COOL GIANT MOLECULAR COMPLEXES(Proceedings of Japan-France Seminar on Chemical Evolution of Galaxies with Active Star Formation)

The interstellar medium is observed to be fragmented at all scales ranging from that of the supercloud complexes to that of the protostellar cores, forming a hierarchy which is not self-similar. In the solar neighbourhood, the self-similarity breaks down at a salient scale which we call that of \u27clouds\u27: their mass is several 100M_, their size a few parsecs. The clouds, building blocks of the hierarchy, are supported against gravity by supersonic but subalfvenic turbulence. We show that this turbulence can be fed, via magnetohydrodynamic waves, over several 10^7 years, by a pumping of the orbital kinetic energy of the other clouds within the same complex. We also show that local gravitational instabilities may develop within the gravitationally stable clouds and eventually form dense protostellar cores