New line-driven wind mass-loss rates for OB stars with metallicities down to $0.01\,Z_\odot$

Abstract: We provide new line-driven wind models for OB stars with metallicities down to $0.01\,Z_\odot$. The models were calculated with our global wind code METUJE, which solves the hydrodynamical equations from nearly hydrostatic photosphere to supersonically expanding stellar wind together with the equations of statistical equilibrium and the radiative transfer equation. The models predict the basic wind parameters, namely, the wind mass-loss rates and terminal velocities just from the stellar parameters. In general, the wind mass-loss rates decrease with decreasing metallicity and this relationship steepens for very low metallicities, $Z\lesssim0.1\,Z_\odot$. Down to metallicities corresponding to the Magellanic Clouds and even lower, the predicted mass-loss rates reasonably agree with observational estimates. However, the theoretical and observational mass-loss rates for very low metallicities exhibit significant scatter. We show that the scatter of observational values can be caused by inefficient shock cooling in the stellar wind, which leaves a considerable fraction of the wind at too high temperatures with waning observational signatures. The scatter of theoretical predictions is caused by a low number of lines that effectively accelerate the wind at very low metallicities.