A photodissociation model for the morphology of the HI near OB associations in M33

We present an approach for analysing the morphology and physical properties of Hi features near giant OB asso- ciations in M33, in the context of a model whereby the Hi excess arises from photodissociation of the molecular gas in remnants of the parent Giant Molecular Clouds (GMCs). Examples are presented here in the environs of NGC604 and CPSDPZ204, two prominent Hii regions in M33. These are the first results of a detailed analysis of the environs of a large number of OB associations in that galaxy. We present evidence for diffusion of the far-UV radiation from the OB association through a clumpy remnant GMC, and show further that enhanced CO(1-0) emission appears preferentially associated with GMCs of higher volume density.