Evolution of massive stars in the Magellanic clouds

Rotation and the associated mixing has recently been included in massive star evolutionary models in an attempt to reconcile theory with observation, e.g surface abundance anomalies, such as nitrogen enrichments. However, despite the apparent importance of rotation there have been few observational tests, specifically in regard to fast rotating stars. This thesis presents the analysis of the Magellanic Cloud B-type stars from the 'The VLT-FLAMES survey of massive stars'. Atmospheric paramaters and rotational velocities have been estimated for rv400 objects, with chemical compositions being derived for over half this sample. This represents the largest homogenous survey undertaken to date. Comparison of the rotational velocity distributions in the SMC, LMC and our Galaxy reveals that objects at low metallicity typically rotate faster as predicted. Additionally the velocity distributions imply that the end of the core-hydrogen burning phase needs to be extended in current theoretical models. The present-day chemical compositions of the Magellanic Clouds have been derived, demonstrating that simply scaling the solar composition is not appropriate for all elements. Additionally chemical compositions have been estimated for a broad range of rotational velocities and hence are ideal for exploring the theory of rotational mixing. In the SMC and LMC both an excess of nitrogen enriched core hydrogen burning slowly rotating objects and highly enriched supergiants are observed. Additionally a group of LMC objects near the end of core-hydrogen burning with large rotational velocities and little enrichment has been identified. These observations are all incompatible with currently adopted rotational mixing theories. The work in this thesis highlights the need for examining entire populations of objects in order to make meaningful and robust comparisons with theory as well as providing a significant advance in our knowledge of massive star evolution.EThOS - Electronic Theses Online ServiceGBUnited Kingdo