Electronographic polarimetry of reflection nebulae

Abstract

In the first part of this thesis imaging polarimeter developed at the University of Durham for use in the optical waveband is described, and details are presented of a new computer data reduction system which allows the production of polarisation naps automatically from polarimeter images recorded using a McMullan electronographic camera. New methods arc employed to achieve higher accuracy and reliability than previously possible, particularly at low brightness levels. A method of checking the smoothness and mutual consistency of images is described, and its use in eliminating image defects is illustrated. This technique is also used to investigate the sources of error in electronography and ways are suggested in which these errors can be reduced, In the second part of the thesis, new polarimetric observations of the reflection nebula NGC1999 arc presented and discussed' in the light of previous colour and brightness observations. Theoretical arguments are used to relate important nebular parameters to observational features in reflection, nebulae, and these ideas are applied to produce a numerical model of NGC1999. Complete multiple scattering calculations are performed and the model is shown to be consistent with all available observations of the nebula. Finally, observations of other reflection nebulae are discussed and the experience gained with NGC1999 is used to outline deficiencies in current models of the interstellar extinction law, and it is suggested that a broad spectrum of particle sizes in the interstellar medium may be indicated. The .importance of imaging polarimetry in further investigation of this problem is pointed out.