Physical Properties of Wolf-Rayet Stars at Infra-red Wavelengths

Wolf-Rayet (WR) stars represent the ultimate phase of evolution for the most massive stars in the Universe. Hot and luminous - they drive dense outflows, giving rise to rich emission-line spectra featuring nitrogen, carbon, and/or oxygen, as deeper layers of nuclear-processed material are revealed. This stripped nature implicates them as Type Ib/c supernovae progenitors, yet how the majority reach this state is unclear. The standard view of line-driven mass-loss producing WR stars is seceding to binary processes. The goal of this thesis is to combine statistics for the Galactic WR population, with physical properties of specific objects, to assess how well these can be explained by stellar models - of single and multiple massive stars. This has been achieved through observations in the infra-red - an increasingly important wavelength regime, abetted by low interstellar extinction and rapidly advancing instrumentation. I present a map of 356 Galactic WR stars, created using calibrated (1–8μm) absolute magnitudes by spectral subtype, and a refined near-IR classification scheme. I compare WR subtype variations with metallicity to population synthesis outputs, finding little evidence for ubiquitous fast stellar rotation. I produce a toy model of the total Galactic WR population using spatial information gleaned. Oxygen abundances in 7 WC and WO stars are determined using Herschel PACS scans of [OIII]88.36μm. These are combined with other recent analyses to argue for a reduction in the 12C(α,γ)16O reaction rate in stellar models. I present a spectroscopic analysis of the largest coeval population of WR stars in the Galaxy - that of the Westerlund 1 cluster. The youth of this cluster prohibits <40 Msun progenitors, hence the physical properties derived - particularly low luminosity - suggest a binary origin for most.