Imaging polarimetry of cometary nebulae
In this work a review of many of the current theories of star formation as it is understood today is presented. New polarimetric observations of three cometary nebulae, RMon/NGC2261, RCr A/NGC6729 and the Chamaeleon Infrared Nebula are presented and discussed. It is shown how previous polarimetric measurements of the illuminating source of Hubble's variable nebula (NGC2261) have often produced results which increase in error with increasing wavelength. The reason for this is that previous authors have used an aperture size for measurements of R Mon which includes effects of a highly polarized feature ~ 5" north of RMon. Though this phenomenon has been seen before by other observers, its effect on polarimetric measurements of R Mon has not been recognised before despite tests to check for this. The data presented here agree with the interpretation that this feature is the northern-lobe of a mini-bipolar nebula, and it is further suggested that this is a manifestation of episodic mass outflow from R Mon. Previous explanations for the polarization of R Mon and RCr A cannot explain the rapid change in polarization and position angle that these young stellar objects are seen to undergo. Models of these objects which assume that they appear as polarized sources are used to explain the polarizations and are discussed. These models are not only able to produce the level of polarization seen at the source, but they are also currently the best models for explaining the rapid changes in polarization that are observed. A jet-like feature is seen to the south-west of the main nebulosity in the Chamaeleon IRN for the first time in observations presented in this thesis. A similar jet seen emerging from RCr A has been explained as being an emission feature collimated by an inner-circumstellar disc by previous authors. Evidence presented in this work for the jets seen emanating from both RCr A and the Chamaeleon IRN, suggests that these features are merely the preferentially illuminated rims of one of the outflow cavities, seen mainly by reflection of light from the source. Further evidence is provided to show that NGC2261 and NGC6729 are illuminated by RMon and RCr A respectively. In the case of the Chamaeleon IRN new evidence is provided to show that the nebula is illuminated by a heavily obscured infrared source located midway between the two outflow cavities.