Predictions for mass-loss rates and terminal wind velocities of massive O-type stars

TL;DR

This study predicts mass-loss rates and wind velocities for massive O-type stars using advanced models, revealing a fundamental limit for stars below a certain luminosity and confirming the accuracy of existing prescriptions for more luminous stars.

Contribution

It introduces a new grid of predictions for O star winds and highlights a fundamental luminosity threshold affecting wind initiation, testing and validating radiation-driven wind models.

Findings

Models fail for stars below log(L/Lsun)=5.2 due to insufficient momentum.

The boundary between wind initiation and failure is at spectral type O6/O6.5.

Predicted mass-loss rates agree with Vink et al. 2000 for luminous stars.

Abstract

Mass loss forms an important aspect of the evolution of massive stars, as well as for the enrichment of the surrounding ISM. Our goal is to predict accurate mass-loss rates and terminal wind velocities. These quantities can be compared to empirical values, thereby testing radiation-driven wind models. One specific issue is that of the "weak-wind problem", where empirically derived mass-loss rates fall orders of magnitude short of predicted values. We employ an established Monte Carlo model and a recently suggested new line acceleration formalism to solve the wind dynamics consistently. We provide a new grid of mass-loss rates and terminal wind velocities of O stars, and compare the values to empirical results. Our models fail to provide mass-loss rates for main-sequence stars below a luminosity of log(L/Lsun) = 5.2, where we run into a fundamental limit. At luminosities below this critical value there is insufficient momentum transferred in the region below the sonic point to kick-start the acceleration. This problem occurs at the location of the onset of the weak-wind problem. For O dwarfs, the boundary between being able to start a wind, and failing to do so, is at spectral type O6/O6.5. The direct cause of this failure is a combination of the lower luminosity and a lack of Fe V lines at the wind base. This might indicate that another mechanism is required to provide the necessary driving to initiate the wind. For stars more luminous than log(L/Lsun) = 5.2, our new mass-loss rates are in excellent agreement with the mass-loss prescription by Vink et al. 2000. This implies that the main assumption entering the method of the Vink et al. prescriptions - i.e. that the momentum equation is not explicitly solved for - does not compromise the reliability of the Vink et al. results for this part of parameter space (Abridged).