Star Forming Giant Molecular Clouds

Abstract

The properties of galactic giant molecular clouds (GMCs) in the solar vicinity and in the inner Galaxy are reviewed. Special attention is given to the role of the clouds in forming stars. The question of whether all GMCs form stars is raised and it is shown that there is little evidence that GMCs anywhere in the Galaxy are devoid of star formation, even O star formation. The angular momentum of local GMCs is then discussed. It is shown quantitatively that the specific angular momentum of GMCs is within the range expected if the clouds condense out of the diffuse interstellar medium. At least four GMCs in the solar neighborhood, however, have retrograde rotation in an inertial frame of reference, placing significant constraints on how they could have formed. Three GMCs in different evolutionary states are identified, and and some of the differences in their properties are identified. GMCs are shown to have atomic envelopes with masses comparable to their molecular masses. These envelopes are likely to pervade the interclump medium and be responsible for most of its mass.

The structure of GMCs is then discussed in some detail. It is argued that clumps are the fundamental units in which the GMC mass is collected. Clumps are shown to have a small volume filling fraction, and that the clump-interclump density contrast is high. The kinematics of the clump ensemble in one GMC, the Rosette Molecular Cloud, is shown to be consistent with the idea that they have have undergone dynamical evolution; the most massive clumps are concentrated toward the midplane of the complex and also possess a lower clump-to-clump velocity dispersion. The clump mass spectrum for five GMCs is compared and shown to be remarkably similar; it is a power law with an index of -1.6 over more than three orders of magnitude in mass with a spread from cloud to cloud of only 10%. Although the majority of clumps do not appear to be gravitationally bound, these clumps can be confined by the pressure of an atomic intercloud medium. The observations presented in this review are used to suggest a tentative outline for the evolution of GMCs.