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تسجيل مستخدم جديدCritical angular velocity and anisotropic mass loss of rotating stars with radiation-driven winds
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The understanding of the evolution of early-type stars is tightly related to that of the effects of rapid rotation. For massive stars, rapid rotation combines with their strong radiation-driven wind. The aim of this paper is to investigate two questions that are prerequisite to the study of the evolution of massive rapidly rotating stars: (i) What is the critical angular velocity of a star when radiative acceleration is significant in its atmosphere? (ii) How do mass and angular momentum loss depend on the rotation rate? To investigate fast rotation, which makes stars oblate, we used the 2D ESTER models and a simplified approach, the $omega$-model, which gives the latitudinal dependence of the radiative flux in a centrifugally flattened radiative envelope. We find that radiative acceleration only mildly influences the critical angular velocity, at least for stars with masses lower than 40 Msun. We explain this mild reduction of the critical angular velocity compared to the classical Keplerian angular velocity by the combined effects of gravity darkening and a reduced equatorial opacity that is due to the centrifugal acceleration. To answer the second question, we first devised a model of the local surface mass flux, which we calibrated with previously developed 1D models. The discontinuity (the so-called bi-stability jump) included in the $dot{M}-T_{rm eff}$ relation of 1D models means that the mass flux of a fast-rotating star is controlled by either a single wind or a two-wind regime. Mass and angular momentum losses are strong around the equator if the star is in the two-wind regime. We also show that the difficulty of selecting massive stars that are viewed pole-on makes detecting the discontinuity in the relation between mass loss and effective temperature also quite challenging.
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[Abridged] Context: Radiation-driven mass loss plays a key role in the life-cycles of massive stars. However, basic predictions of such mass loss still suffer from significant quantitative uncertainties. Aims: We develop new radiation-driven, steady-
state wind models for massive stars with hot surfaces, suitable for quantitative predictions of global parameters like mass-loss and wind-momentum rates. Methods: The simulations presented here are based on a self-consistent, iterative grid-solution to the spherically symmetric, steady-state equation of motion, using full NLTE radiative transfer solutions in the co-moving frame to derive the radiative acceleration. We do not rely on any distribution functions or parametrization for computation of the line force responsible for the wind driving. Results: In this first paper, we present models representing two prototypical O-stars in the Galaxy, one with a higher stellar mass M/Msun=59 and luminosity log10(L/Lsun)= 5.87 and one with a lower M/Msun= 27 and log10(L/Lsun)= 5.1. For these simulations, basic predictions for global mass-loss rates and velocity laws are given, and the influence from additional parameters like wind clumping and microturbulent speeds discussed. A key result is that our mass-loss rates are significantly lower than those predicted by the mass-loss recipes normally included in models of massive-star evolution. Conclusions: Our results support previous suggestions that Galactic O-star mass-loss rates may be overestimated in present-day stellar evolution models, and that new rates thus might be needed. Indeed, future papers in this series will incorporate our new models into such simulations of stellar evolution, extending the very first simulations presented here toward larger grids covering a range of metallicities, B supergiants across the bistability jump, and possibly also Wolf-Rayet stars.
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