The origin of ejecta nebulae surrounding massive stars

Luminous Blue Variables (LBVs) are a rare class of stars which represent a phase in the transition of Of stars into Wolf-Rayet stars. A ubiquitous feature of LBVs is the presence of a circumstellar nebula. These nebulae bare witness to a phase of enhanced mass loss, the gross characteristics and causes of which are basically unknown. In this thesis we analyse the chemical and dynamical properties of these nebulae to infer the physical conditions present on the surface of the star at the epoch of ejection. We have undertaken abundance studies of three LBV nebulae (AG Car, HR Car and He3-519) and found them to consist of partially CNO processed material. Since the application of atmospheric models to LBV stars has shown them to have a CNO equilibrium abundance pattern, this suggests that the epoch of nebular formation occurred before the present LBV phase. From the low expansion velocities of LBV nebulae we infer that the nebulae were ejected from stars with a low surface gravity. We note that this could arise either when a star approaches its limit or during a RSG phase. Since mass loss near the ? limit is inherently asymmetric and LBV nebulae are in general bipolar, we favour the former possibility. In a dynamical study of the nebula surrounding AG Car we find that it has a prolate geometry, with a disk-like outflow external to the nebula. Since these are hallmarks of mass loss from stars nearing the limit, this reinforces our belief that the mass loss occurred toward the blue. In addition we have performed abundance studies on the nebula RCW 58 which surrounds the WR star WR 40 and NGC 6164/5 surrounding the Of star HD 148937. We find that both these nebulae have very similar abundance patterns to those nebulae surrounding LBV stars. This suggests that the surface abundance pattern plays an important role in the operation of the ejection mechanism. In this context we note the significance of the surface abundance pattern for the opacity of the outer layers which govern the star's interaction with its modified Eddington limit.