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RADIAL VELOCITIES in the ZODIACAL DUST CLOUD

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RADIAL VELOCITIES in the ZODIACAL DUST CLOUD A SURVEY OF RADIAL VELOCITIES in the ZODIACAL DUST CLOUD Brian Harold May Astrophysics Group Department of Physics Imperial College London Thesis submitted for the Degree of Doctor of Philosophy to Imperial College of Science, Technology and Medicine London · 2007 · 2 Abstract This thesis documents the building of a pressure-scanned Fabry-Perot Spectrometer, equipped with a photomultiplier and pulse-counting electronics, and its deployment at the Observatorio del Teide at Izaña in Tenerife, at an altitude of 7,700 feet (2567 m), for the purpose of recording high-resolution spectra of the Zodiacal Light. The aim was to achieve the first systematic mapping of the MgI absorption line in the Night Sky, as a function of position in heliocentric coordinates, covering especially the plane of the ecliptic, for a wide variety of elongations from the Sun. More than 250 scans of both morning and evening Zodiacal Light were obtained, in two observing periods – September-October 1971, and April 1972. The scans, as expected, showed profiles modified by components variously Doppler-shifted with respect to the unshifted shape seen in daylight. Unexpectedly, MgI emission was also discovered. These observations covered for the first time a span of elongations from 25º East, through 180º (the Gegenschein), to 27º West, and recorded average shifts of up to six tenths of an angstrom, corresponding to a maximum radial velocity relative to the Earth of about 40 km/s. The set of spectra obtained is in this thesis compared with predictions made from a number of different models of a dust cloud, assuming various distributions of dust density as a function of position and particle size, and differing assumptions about their speed and direction. The observations fit predominantly prograde models fairly well, but show a morning-evening asymmetry, different in the two observing periods. Models are investigated containing various components, including prograde and retrograde orbiting dust around the Sun, a drift of interstellar material though the Solar System, and light from distant emitting matter. The implications for possible asymmetries of the Zodiacal Cloud are discussed. Other researches on the Zodiacal Dust Cloud, before, during, and after my observations, are reviewed, including recent insights into its structure, orientation, and evolution, up to the present day, and my observations are evaluated in this context. Period of study, 1970-2007. 3 Contents Abstract 2 Declaration and Copyright 7 Dedication 8 Preface and Acknowledgement 9 List of Figures 14 Abbreviations and Acronyms 17 1 Introduction 18 1.1 The Zodiacal Light – the phenomenon . 18 1.2 Review of observations of the ZL prior to 1970 . 19 1.2.1 Earliest views . 19 1.2.2 Evidence as to the scattering medium . 26 1.2.3 Measurable quantities in ZL observations . 27 1.2.4 Physical parameters and models of the ZL . 29 1.2.5 Doppler investigations of the ZL prior to this study . 33 1.3 Review of the period of my observations of the ZL, 1971-1974 . 41 1.4 Review of recent developments in ZL research, 1974-2007 . 42 1.4.1 Overview . 42 1.4.2 Infrared surveys . 46 1.4.2.1 Modelling based on IRAS data . 46 1.4.2.2 ISO . 54 1.4.2.3 COBE . 57 1.4.3 Collected samples and in situ measurements . 58 1.4.4 Interstellar dust . 60 1.4.5 Doppler measurements and models since 1974 . 61 2 Preparations and experimental details 1971-1974 64 2.1 Choice of wavelength for the observations . 64 2.2 Choice of resolving power . 66 2.3 The site . 66 2.4 The ZL Building . 68 2.5 Instrumental details . 69 4 2.5.1 The coelostat and sky area accessible . 69 2.5.2 The Fabry-Perot interferometer (F-P) . 72 2.6 New features of the instrument . 76 2.6.1 Fore-optics . 76 2.6.2 Field lens . 76 2.6.3 Beam splitter for compensation channel . 77 2.6.4 Construction of housing . 78 2.6.5 Fabry-Perot theory, and the finesse of a practical interferometer . 79 2.6.5.1 Reflection finesse . 80 2.6.5.2 Aperture finesse . 83 2.6.5.3 Defect finesse . 85 2.6.5.4 Coating non-uniformities . 87 2.6.5.5 The choice of plates . 89 2.6.6 Intensity calibration box . 92 2.6.7 Wavelength calibration assembly . 93 2.6.8 Control electronics . 94 2.6.9 Output devices . 99 2.7 Optimisation of the system . 99 2.7.1 Optics . 99 2.7.2 Counting electronics . 100 2.7.3 The system as a whole . 100 2.8 How the observations were made . 101 3 Reduction of the Data 105 3.1 Intensity calibration . 105 3.1.1 Method and calculations . 105 3.1.2 Accuracy of intensity calibration . 109 3.2 Wavelength calibration . 110 3.2.1 Determination of the datum line position . 110 3.2.2 Establishment of wavelength scale . 111 3.3 Treatment of a single scan . 114 3.4 Daylight correction . 117 3.5 MgI emission . 119 5 3.5.1 Discovery . 119 3.5.2 Wavelength of emission . 121 3.5.3 Intensity of the emission . 121 3.5.4 Variation of daily average emission . 123 3.5.5 Variation of emission throughout the night . 124 3.5.6 Final comments on MgI emission . 136 3.6 Averaging . 127 3.7 Polynomial fits . 128 3.7.1 General . 128 3.7.2 Computation of the minimum point . 131 3.7.3 Computation of shift by the ‘inflexion points’ method . 133 3.7.4 Assessment of these methods . 135 3.8 Gaussian Curve Fitting . 136 3.8.1 Computation . 136 3.8.2 Experimental case for the use of Gaussian fits . 136 3.8.3 Theoretical justification for the use of Gaussian fits . 137 4 Interpretation of results in terms of physical models 140 4.1 Gallery of fully reduced scans . 140 4.2 General comments on the data . 151 4.3 Information obtainable from the Gaussian fits . 152 4.4 Accuracy of the fits . 156 4.5 Evidence from line-widths and depths . 157 4.6 Analysis of the wavelength versus elongation data . 159 4.7 Forces experienced by particles in the Zodiacal Cloud . 159 4.8 Rotating dust cloud models . 160 4.9 Other considerations in rotating models . 163 4.10 Geocentric Dust Cloud (GDC) model . 164 4.11 A uniform dust flow model . 166 4.11.1 Uniform flow theory . 167 4.11.2 Comparison of continuous flow model with experiment . 175 4.12 Estimation of the effect of a contribution to the Zodiacal Light from sources distant from the Solar System . 176 4.12.1 Computation . 176 6 4.12.2 Comparison of distant emitting model with the data . 180 4.13 Further analysis of the data by comparison with new radial velocity data and model theory available.
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