Abbott, J.B.; (2004) Quantitative spectroscopic studies of Wolf-Rayet stars in local group galaxies. Doctoral thesis , University of London.

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Abstract

With recent advances in astronomical instrumentation, individual massive stars in Local Group galaxies are now easily accessible to ground-based telescopes. We present deep, high quality images for two late-type spiral galaxies; NGC300 and M83, located beyond the Local Group at distances in excess of a Mpc. Through the use of narrow-band interference filters, utilizing the strong Wolf-Rayet (WR) emission lines, we have identified a large number of previously unknown WR stars revealing a significant increase in the known WR population of these galaxies, resulting in new catalogues. We have also used a number of modern ground-based optical telescopes with efficient multi-object spectrographs to obtain observations for a representative sample of known extra-galactic WR stars in M33, M31 and IC10, significantly improving on existing data. These galaxies are members of the Local Group and span a metallicity range of approximately a factor of 10, providing us with a variety of environments in which to study individual WR stars. Our large sample has allowed us to re-examine a number of spectral morphologies and to asses their potential multiplicity. We have also re-evaluated the metallicity gradients for three spiral galaxies in which we have observed WR stars, M33, M31 and NGC300 - the results of which are considered in later analysis. For our sample of identified single WR stars we perform detailed, tailored analyses using the non-LTE code, CMFGEN, which assumes a spherically expanding, extended atmosphere subject to line blanketing. Fundamental stellar parameters are derived for each star and subsequently collated, permitting us to investigate the role that metallicity plays on the derived stellar properties for both WN and WC subtypes. The WN stars examined display little difference in the observed properties between the Galactic and LMC samples. Comparison of the single WN stars in the SMC suggest they have significantly reduced mass-loss rates and increased luminosities, implying that WN stars display an observational mass-loss -metallicity relationship comparable to O-type stars (i.e. M alpha Z0.5-0.8). Results for our sample of extra-galactic WC stars appear somewhat less clear, with a range of derived mass-loss rates and luminosities for stars located in a variety of environments. Our results imply that early-type WC stars are relatively uniform and that their wind structure is inherently the same, as indicated through their constant observed line fluxes, after correcting them for distance, and their narrow range of derived properties. Our results call into question whether their mass-loss rates are enhanced with increasing metallicity or whether other factors are important; such as carbon and oxygen significantly aiding line driving and further accelerating the star's atmosphere. Further analysis of more high quality spectroscopic observations are required, for both subtypes of WR stars, located in as many extreme environments as possible to confirm our conclusions.

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