Critical angular velocity and anisotropic mass loss of rotating stars with radiationdriven winds
Abstract
Context. The understanding of the evolution of earlytype stars is tightly related to that of the effects of rapid rotation. For massive stars, rapid rotation combines with their strong radiationdriven wind.
Aims: 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?
Methods: To investigate fast rotation, which makes stars oblate, we used the 2D ESTER models and a simplified approach, the ωmodel, which gives the latitudinal dependence of the radiative flux in a centrifugally flattened radiative envelope.
Results: We find that radiative acceleration only mildly influences the critical angular velocity, at least for stars with masses lower than 40 M_{☉}. For instance, a 15 M_{☉} star on the zeroage main sequence would reach criticality at a rotation rate equal to 0.997 the Keplerian equatorial rotation rate. 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 socalled bistability jump) included in the Ṁ  T_{eff} relation of 1D models means that the mass flux of a fastrotating star is controlled by either a single wind or a twowind regime. Mass and angular momentum losses are strong around the equator if the star is in the twowind regime. We also show that the difficulty of selecting massive stars that are viewed poleon makes detecting the discontinuity in the relation between mass loss and effective temperature also quite challenging.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 May 2019
 DOI:
 10.1051/00046361/201834599
 arXiv:
 arXiv:1904.05089
 Bibcode:
 2019A&A...625A..88G
 Keywords:

 stars: rotation;
 stars: massloss;
 stars: earlytype;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 16 pages, accepted in Astronomy and Astrophysics