Structural Model of Molecular Cloud Complexes: Mass, Size, and External Pressure

We investigate the structure of the molecular cloud complexes (MCCs) as a group of several giant molecular clouds (GMCs) in the Galaxy. Then, we find that the mass-size relation which has been reported for the GMCs establishes well even for the very large MCCs whose size is about 1 kpc. Since the horizontal size of the MCCs is more than the thickness of the Galactic disk, we can no longer consider the MCCs to be a sphere. Thus, we construct a structural model of the MCCs, adopting a rectangular-solid geometry. As a result, our model explains the observed mass-size relation of the MCCs very well. {}From the estimated external pressure around the MCCs, we find that they are in a rough pressure balance with the interstellar medium. Moreover, we find there is observational deficiency of the MCCs with a large size and surface density. Then, we suggest that the external pressure has a significant effect on the structure and evolution of the MCCs. We also discuss the effect of H {\sc ii} regions in the MCCs.