Observations related to the possibility that there are different mechanisms of formation for O-type stars and later type stars

Carbon monoxide observations of neutral matter near W3 provide an excellent example of the formative process of 0 stars. As in other regions where O stars are forming the young stars in W3 are located in dense cloud fragments that are at the edge of an extended molecular cloud complex. This peculiar formation site is simply explained in terms of a million year old shock that probably surrounds the expanding H II region IC1805. The contrast between this situation for W3 and what is found in other star-forming regions where only low mass stars occur is emphasized. (Auth.)