A study of extended red emission in reflection nebulae

Abstract

In the first part of this thesis spectroscopic observations of the Red Rectangle nebula are presented. A link is made between the most prominent unique emission line features, which are superimposed on the broad emission spectrum of the Red Rectangle, with the absorption features of the diffuse interstellar bands (DIBs). The shape of the emission lines are shown to be temperature dependent with the bandwidth decreasing by 4Å and the the peak wavength altering by 5Å in the cooler outer parts of the nebula when compared with the hotter inner regions. Both peak wavelength and bandwidth can be extrapolated to the values of the associated DIBs. A possible common carrier is discussed for the absorption/emission line pairs. In the second part polarimetric observations of two reflection nebulae, NGC 7023 and NGC 2023, are presented in the B, V, R, I, and V, R, I wavebands respectively. Through the use of colour and polarization colour diagrams a search was made for intrinsic emission, also known as extended red emission (ERE). Two such regions were found in NGC 7023, one 64"S and the other 53"N of HD 200775, in which there was a reduction in the R and I polarizations when compared with the values expected from scattering alone together with an increase in reddening. Calculations show that the ERE contributes as much as 22% of the light in the R waveband and approximately 16% in the I waveband. For NGC 2023 two suspected regions were investigated, the first 62"ENE and the second 84"ENE of HD 37903. It was found that the region nearest to the central star was unlikey to be a region of ERE however the second region exhibited a decrease in the R polarization together with an increase in R intensity but this was not shown to be the case in the I waveband. Several explanations are provided to explain this behaviour. The ERE is thus shown to be localised and varying in quantity between different positions in a nebula. None of the emission line features prominent in the Red Rectangle were found in the spectrum of the filament 84"ENE of HD 37903 and the implications of this are discussed.