The optical polarisation of m42 and m43

Abstract

This thesis contains a description of the technique used to measure the optical linear polarisation of the Orion Nebula and the nebula, M43, and the results obtained from these measurements. This work was undertaken while the author was a member of the Durham University Astronomy Group. The first chapter is an outline of the differing states of polarisation and the method used to describe these states via the Stokes Parameters. A short review of the mechanisms producing linear polarisation of the optical radiation from astronomical objects is included. Chapter 2 describes briefly the mechanisms producing plane polarisation and dust scattering and also contains a description of Rayleigh and Mie Scattering. Chapter 3 contains a brief account of the design and operation of the Durham University nebular polarimeter used to detect and measure optical linear polarisation, along with a short description of the electronographic camera used as a recording device. Also, the technique for the reduction of the data, from its initial electronograph to the final extraction of the Stokes parameters for over 8550 points in the Orion Nebula is described. The next chapter contains an account of gaseous nebulae, particularly the Orion Nebula, where a review of the present properties of this object and some general remarks on M43 are made. In Chapter 5 of this thesis, the technique employed to measure the optical linear polarisation of these nebulae, together with a final map of the polarisation for each of them, and a comparison of the results obtained by the present method for the Orion Nebula with those measurements of Hall (1974) and Pallister et al (1977) are described which confirm the success of the present technique. In the following chapter, some interpretation of the results and the geometry of the Orion Nebula and M43 are described. Also a simple model of the Orion Nebula as a spherically symmetric dust cloud with 0(^1)ori at the centre is presented. A comparison between his model considering the constant dust density case, and the polarisation measurements, is made in this chapter and some possible causes of the disagreement between this model and observational results are presented. In conclusion, the success of the technique is confirmed for the production of high resolution, accurate polarimetric measurements.