Evolution of line-force multiplier parameters in radiation driven winds of massive stars



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Universidad de Valparaíso





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Instituto de Fisica y de Astronomia




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Massive stars expell strong stellar winds which are described by the theory of radiation-driven wind. Accurate mass-loss rates are needed to properly describe the stellar evolution across the Hertzsprung-Russel Diagram. We present two self-consistent procedures that couple the hydrodynamics with calculations of the line-force in the frame of radiation wind theory. These procedures give us the lineforce parameters, the velocity field, and the mass-loss rate. The first one is based on the so-called m-CAK theory. Such computations contemplate the contribution to the line-force multiplier from more than ∼ 900, 000 atomic transitions, an NLTE radiation flux from the photosphere and a quasi-LTE approximation for the occupational numbers. A full set of line-force parameters for Teff ≥ 32, 000 K and surface gravities higher than 3.4 dex for two different metallicities are presented, along with their corresponding wind parameters (terminal velocities and mass-loss rates). Here, we find that the already known dependence of lineforce parameters on effective temperature is enhanced by the dependence on log g. Terminal velocities present a steeper scaling relation with respect to the escape velocity, this might explain the scatter values observed in the hot side of the bistability jump. For the case of homogeneous winds (i.e., without clumping) comparison of self-consistent mass-loss rates shows a good agreement with empirical values. We also consider self-consistent wind solutions that are used as input in FASTWIND to calculate synthetic spectra. By comparison with the observed spectra for three stars with clumped winds, we found that varying the clumping factor the synthetic spectra rapidly converge into the neighbourhood region of the solution. Therefore, this self-consistent m-CAK procedure significantly reduces the number of free parameters needed to obtain a synthetic spectrum. The second procedure (called Lambert-procedure) provides a self-consistent solution beyond m-CAK theory and its approximations, and line-acceleration is calculated by the full NLTE radiative transfer code CMFGEN. Both the mass-loss rate and the clumping factor are set as free parameters, hence their values are obtained by spectral fitting after the respective self-consistent hydrodynamics is calculated. Since performing the Lambert-procedure requires significant computational power, the analysis is made only for the star ζ-Puppis. It is found that fitted wind-parameters are close to those predicted by the m-CAK prescription. This suggests that both methodologies providing a lower clumping effect on the wind that those 11 12 CONTENTS suggested by previous authors. We illustrate the future potential of the self-consistent m-CAK prescription, showing the first results of two ongoing works: the spectral fitting for a set of high resolution spectra observed by Hermes and the development of new evolutionary tracks with the Geneva evolutive code using self-consistent mass-loss rates. The promising results gives a positive balance about the future applications for the self-consistent solutions presented on this thesis.


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