The Baldwin-effect in Wolf-Rayet stars

Abstract

This thesis investigates the Baldwin-effect in Wolf-Rayet (WR) stars. The Baldwin-effect is a correlation with negative slope between the equivalent width of spectral emission lines and the monochromatic luminosity of the underlying continuum at the line wavelength. This effect has been known to exist in spectra of Active Galactic Nuclei and WR stars. The Baldwin effect in WR stars may function as a distance estimator, as it may provide a method to derive the stellar luminosity directly from the spectrum. Distances of WR stars are hard to derive. These stars lose matter in the form of a strong stellar wind, which hides the underlying star from view. The dense stellar wind is the reason that absolute visual magnitudes of WR stars do not correlate well with the star's temperature or intrinsic colours. Therefore, the derivation of the distance of a WR star through observational properties is not an easy task to perform. The research of this thesis confirms previous thoughts that the Baldwin effect in WR stars is mainly caused by differences in radius among stars, reflecting differences in wind density. Furthermore, it is found that the spread in the equivalent width-luminosity relations is caused by diffences in mass-loss rate and terminal wind velocity among stars and, to a lesser extend, differences in stellar temperature. This theoretical explanation of the Baldwin-effect is done by comparing simple semi-analytical expressions with the results of model stellar atmosphere calculations. For the application of the Baldwin-effect to estimate WR star distances, it is found that a reasonably accurate estimate of the mass-loss rate is required. However, the derived mass-loss rates of WR stars generally depend on an assumed distance of the star. Therefore it is tried to combine several mass-loss studies and find typical values of the mass-loss rate as function of stellar subtype. After adopting a value of the mass-loss rate for its spectral subtype, the distance of the star can be estimated through the Baldwin-effect. The performance of the Baldwin-effect as a distance estimator is tested by concentrating at WR stars with well known distances, such as those in the Large Magellanic Cloud or WR stars that are a member of an open star cluster or OB association. It is concluded that estimating WR star distances by means of this method does give a reasonable indication of the stellar distance, but often not better than existing methods.