Galactic Wolf-Rayet stars with Gaia: Origins, Properties and Extragalactic Applications

Wolf-Rayet (WR) stars are the helium core burning descendants of massive O stars. Their strong and dense stellar winds remove outer hydrogen layers from the atmosphere, revealing core burning products, which appear as broad emission lines. As the final evolutionary phase of high mass stars, they can reveal much about massive star origins, evolution and fates. In this thesis, we study Galactic and extragalactic WR stars using results from Gaia Data Release 2, which contains parallaxes and proper motions for >1 billion Galactic stars. We find distances for 383 Galactic WR stars, of which 187 have reliable distances and absolute magnitudes at optical or IR wavelengths. Absolute magnitudes were used to calculate subtype calibrations, which can be applied to estimate distances to other WR stars. Additionally, we find 31 potential runaways by searching for stars far from the Galactic plane. We also explore WR star origins, using membership of Galactic clusters, associations and star forming regions. At least 59% of WR stars are probably isolated. Simulations suggest they likely result from sparse (~10 Msun pc^3) associations, which dissolve to field densities (~1-10 Msun pc^3) during the WR star lifetime. Many massive stars may therefore form outside cluster environments. We also determine age estimates of clusters containing WR stars. New distances allow us to determine emission line luminosity calibrations at 4600-4700Ang and 5600-5900Ang . These can be applied to determine the numbers and subtypes of WR stars within unresolved regions of external galaxies. Finally, we surveyed the WR population of the spiral galaxy NGC6946 with Gemini GMOS. We find 92 photometric candidates and confirm 61 WC stars spectroscopically using templates. These results are consistent with populations derived from emission line calibrations. We use spectroscopic nebular data to estimate the central metallicity (12+log(O/H)=8.76+/-0.2) and extinction, AV =1.88+/-0.09 towards the galaxy, consistent with other studies.