New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars: I. Method and first results

Abstract

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 non-local thermodynamic equilibrium 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. The models start deep in the subsonic and optically thick atmosphere and extend up to a large radius at which the terminal wind speed has been reached. Results. In this first paper, we present models representing two prototypical O-stars in the Galaxy, one with a higher stellar mass M∗ Mpdbl = 59 and luminosity log10L∗L = 5.87 (spectroscopically an early O supergiant) and one with a lower M∗Mpdbl = 27 and log10L∗ L- = 5.1 (a late O dwarf). For these simulations, basic predictions for global mass-loss rates, velocity laws, and wind momentum are given, and the influence from additional parameters like wind clumping and microturbulent speeds is discussed. A key result is that although our mass-loss rates agree rather well with alternative models using co-moving frame radiative transfer, they 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 might therefore 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.