| This thesis consists of a study of the density structure within two Galactic molecular clouds, and investigations into their evolution. A method for analyzing spectral line observations of cloud density structure is described in Chapter 2, and the aims and limitations of such an analysis are outlined. In Chapter 3 the method is applied to a proto-typical star forming cloud, the Rosette molecular cloud, and a thorough analysis of its structure is made. Molecular clumps, observed in {dollar}sp{lcub}13{rcub}{dollar}CO, are found to be embedded in a low density interclump medium, observed in HI. Many signs of the cloud's formation and evolution are found. The effect of the previous generation of star formation (the Rosette nebula) is assessed, and the conditions for present and future star formation within the remainder of the cloud are evaluated. Stars form in the most massive, gravitationally bound clumps within the cloud, and with an efficiency that increases with proximity to the HII region. The analysis of the Rosette cloud is used as a baseline to compare with a second molecular cloud, the Maddalena-Thaddeus cloud. This is, at the time of writing, the only massive cloud which is known not to be presently forming any OB stars. The overall, large scale view of the molecular, atomic and associated stellar components of this cloud is shown in Chapter 4: it is shown that there is associated massive star formation that is interacting with the cloud, but that the bulk of the molecular material is non-star forming and in a very different evolutionary state to the Rosette cloud. {dollar}sp{lcub}13{rcub}{dollar}CO(1-0) observations of the density structure within the cloud are described in Chapter 5 and compared with the earlier results for the Rosette cloud. Structurally, the two clouds share many similarities, for example, the clump mass spectrum, mass-size relation, and isothermal clump profiles. But there are differences of scale between the two: the clumps in the Maddalena-Thaddeus cloud have high linewidths and are all gravitationally unbound. They also have low peak column densities and shallow profiles relative to clumps of similar mass in the Rosette cloud. Further comparisons of the physical conditions in the two clouds are discussed in Chapter 6. Higher transition lines of CO and {dollar}sp{lcub}13{rcub}{dollar}CO and CS, a denser gas tracer, are observed. Kinetic temperatures are higher in the Rosette cloud and the ratio of CS to {dollar}sp{lcub}13{rcub}{dollar}CO emission is stronger. The focus shifts from these two clouds to the overall distribution of molecular clouds in the Galaxy in Chapter 7. Previous survey data is reevaluated and an OB association luminosity function and cloud mass spectrum is derived. A simple theoretical model is then used to calculate a joint distribution of OB associations in clouds, and used to compare with observed cloud-association pairings in the Galaxy. Most clouds with masses comparable to the Rosette or Maddalena-Thaddeus clouds ({dollar}Msim10sp5Msb{lcub}odot{rcub}{dollar}) are predicted to contain only a few O and B stars. It is argued, then, that the Maddalena-Thaddeus cloud is an extreme evolutionary example of a rather common type of molecular cloud. Implications of the observations of its density structure and the comparison with the Rosette cloud are then interpreted in Chapter 8 in terms of an evolutionary scenario for molecular clouds. |
