The spectroscopy of DA white dwarfs at high resolution
thesisposted on 01.04.2014, 14:20 by Nigel P. Bannister
This thesis is concerned with the spectroscopy of hot, hydrogen-rich white dwarfs, and the development of instrumentation with which to further their study. It begins with a review of white dwarf research, and a summary of results from some notable Extreme-Ultraviolet (EUV) missions. The absence of helium in hot hydrogen-rich white dwarfs is introduced, and the need to precisely determine their composition is explained. These issues provide motivation for the Joint Astrophysical Plasmadynamic Experiment (J-PEX). Using normal incidence, multilayer coated optics and a high resolution focal plane detector, this spectrometer offers a substantial improvement in effective area and spectral resolution over current instrumentation. The design and development of a microchannel plate (MCP) detector for J-PEX is discussed, and a study of MCP sensitivity enhancement processes presented. A CsI photocathode is found to offer superior performance in the 225 - 245 Å band covered by J-PEX. The low quantum efficiencies recently measured for MCPs are also discussed. A preliminary analysis of data from the first successful flight of J-PEX, on board a sounding rocket, is described. Techniques are devised to overcome uncertainties in wavelength calibration, leading to production of the highest resolution EUV spectrum currently available for an astronomical object. These data reveal the presence of helium along the line of sight to the white dwarf G191-B2B, and prove the value of the J-PEX design. Data from the Hubble Space Telescope and the International Ultraviolet Explorer are used to search for white dwarfs with highly ionised, non-photospheric absorption features. Three new identifications are made, in the white dwarfs REJ 1738+665, REJ 0558+165 and WD 2218+706. Possible explanations, including absorption by planetary nebulæ, are suggested. The spectra are of longer wavelength, but higher resolution than currently obtainable by J-PEX, and complement the latter instrument by resolving multiple velocity components along the line-of-sight. The importance of these measurements in the context of J-PEX results, is demonstrated.