Structural model of molecular cloud complexes: Mass, size, and external pressure

We investigate the structure of the molecular cloud complexes (MCCs) as a g roup of several giant molecular clouds (GMCs) in the Galaxy. We then find t hat the mass-size relation which has been reported for the GMCs is well-est ablished even for very large MCCs, whose sizes are about 1 kpc. Since the h orizontal size of the MCCs is larger than the thickness of the galactic dis k, we can no longer consider the MCCs to be spherical. 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 t hey are in a rough pressure balance with the interstellar medium. Moreover, we find there is an observational deficiency of the MCCs with a large size and surface density. We thus suggest that the external pressure has a sign ificant effect on the structure and evolution of the MCCs. We also discuss the effect of H II regions in the MCCs.