A model of anisotropic winds from rotating stars for evolutionary calculations

TL;DR

This paper introduces a new formalism for modeling anisotropic stellar winds from rotating stars, accounting for surface property variations and rotation-induced distortion, improving evolutionary calculations.

Contribution

We develop a flexible, physically motivated prescription for mass and angular momentum loss in rotating stars, integrated into 1D stellar evolution models, considering surface anisotropies and bi-stability effects.

Findings

Mass-loss rates are insensitive to rotation for non-bi-stability jump models.

Bi-stability jump effects are smoothed in mass-loss rate evolution.

The new prescription aligns well with more complex models and is easy to implement.

Abstract

Context: The surface properties of rotating stars can vary from pole to equator, resulting in anisotropic stellar winds which are not included in the currently available evolutionary models. Aims: We develop a formalism to describe the mass and angular momentum loss of rotating stars which takes into account both the varying surface properties and distortion due to rotation. Methods: Adopting the mass-loss recipe for non-rotating stars, we assigned to each point on the surface of a rotating star an equivalent non-rotating star, for which the surface mass flux is given by the recipe. The global mass-loss and angular momentum loss rates are then given by integrating over the deformed stellar surface as appropriate. Evolutionary models were computed and our prescription is compared to the currently used simple mass-loss enhancement recipes for rotating stars. Results: We find that mass-loss rates are largely insensitive to rotation for models not affected by the bi-stability jump. For those affected by the bi-stability jump, the increase in mass-loss rates with respect to time is smoothed. As our prescription considers the variation of physical conditions over the stellar surface, the region affected by the bi-stability jump is able to grow gradually instead of the whole star suddenly being affected. Conclusion: We have provided an easy to implement and flexible, yet physically meaningful prescription for calculating mass and angular momentum loss rates of rotating stars in a one-dimensional stellar evolution code which compares favourably to more physically comprehensive models. The implementation of our scheme in the stellar evolution code MESA is available online: https://zenodo.org/record/7437006