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.