The properties of cool DZ white dwarfs.

Over the last few decades it has become clear that metals present within the atmospheres of more than one quarter of white dwarfs signify recent accretion of minor bodies from their planetary systems. Spectral analysis of these metal-polluted white dwarfs allows determination of the accreted body composition, providing the most direct method for measuring the makeup of exoplanetary material. So far, most detailed abundance analyses have mostly been limited to a few systems at a time. In this thesis, I present a sample of 231 cool DZ white dwarfs identified from SDSS spectroscopy. These stars exhibit strong metal lines from multiple elements, permitting detailed abundance analyses of each. Furthermore their low effective temperatures of 9000{4400 K imply corresponding cooling ages of 1{8 Gyr, allowing me to examine some of the oldest planetary systems in orbit of stellar remnants. Across the sample, I found a huge diversity in the metal abundance ratios, with Fe/Ca varying by a factor 100. I developed a simple method for interpreting the rocky geology of the accreted parent bodies, indicating that some were composed of > 80 % crust material, and with > 80 % core material for others. Using the calculated white dwarf ages, I identified a downwards trend of the highest levels of metal pollution for the oldest systems, suggesting their mass reservoirs of exoplanetesimals become depleted on a ' 1 Gyr time scale. Finally, Zeeman split metal lines are found in the spectra of 33 of these systems, with surface magnetic fields in the range 0:25{30 MG. Investigation of this rare combination of metals and magnetism has consequences for the formation of white dwarf magnetic fields, and motivates new research in atomic physics