The detection of remnant planetary systems at white dwarfs allows the end stages of planetary evolution to be explored observationally. This thesis presents observations of white dwarfs and describes the contributions they make to planetary science. Firstly, white dwarf science probes the end states of the majority of known planetary systems, including the Solar system. In Chapter 3 I present the discovery of strongly variable emission lines from gas in a debris disc around the white dwarf SDSS J1617+1620. Time-series spectroscopy obtained during the period 2006–2014 has shown the appearance and then complete disappearance of strong double-peaked Ca ii emission lines. These observations represent unambiguous evidence for short-term variability in the debris environment of evolved planetary systems. Possible explanations for this extraordinary variability include the impact onto the debris disc of a single small rocky planetesimal, or interactions between material in a highly eccentric debris tail. I also use observations of white dwarfs to contribute to exoplanet science more generally. Metal pollution from planetary debris is visible in spectra of white dwarfs, providing the only technique to directly measure the bulk chemical composition of rocky extrasolar material. Chapter 4 presents a detailed study of the metal-polluted white dwarf SDSS J0845+2257, using high-resolution HST/COS and VLT spectroscopy to detect hydrogen and eleven metals originating in an orbiting debris disc. The chemistry of the debris is broadly similar to the Earth, but enhanced abundances of core material (Fe, Ni) suggest that the planetesimal from which the debris formed may have lost a portion of its mantle. Conversely, in Chapter 5 I focus on the detection of just carbon and oxygen, but at 16 different white dwarfs to search for hypothetical “carbon planets”. I find no evidence for carbon-rich planetesimals, with C/O< 0:8 by number in all 16 systems. I place an upper limit on the occurrence of carbon-rich systems at < 17 percent. The range of C/O of the planetesimals is consistent with that found in the Solar System. White dwarfs can also be fascinating objects in their own right. In Chapter 6 I present HST observations of the mysterious white dwarf GD 394, a hot, extremely metal polluted white dwarf. Extreme ultraviolet observations in the mid 90s revealed a 1.15 day periodicity with a 25 per cent amplitude, hypothesised to be due to a surface accretion spot. I obtained phase-resolved HST/STIS high-resolution FUV spectra of GD 394 that sample the entire period, along with a large body of supplementary data. I use these data to test the hypothesis of an accretion spot, search for variability in accretion rates over decades-long timescales, and probe the immediate circumstellar environment of GD 394.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:742221 |
| Date | January 2017 |
| Creators | Wilson, David John |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/101779/ |
Page generated in 0.0015 seconds