2021MNRAS.504.2051V

Mass-loss rates and terminal wind velocities are key parameters that determine the kinetic wind energy and momenta of massive stars. Furthermore, accurate mass-loss rates determine the mass and rotational velocity evolution of mass stars, and their fates as neutron stars and black holes in function of metallicity (Z). Here, we update our Monte Carlo mass-loss Recipe with new dynamically consistent computations of the terminal wind velocity - as a function of Z. These predictions are particularly timely as the Hubble Space Telescope Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) project will observe ultraviolet spectra with blue-shifted P Cygni lines of hundreds of massive stars in the low-Z Large and Small Magellanic Clouds (SMC), as well as sub-SMC metallicity hosts. Around 35 000 K, we uncover a weak-wind 'dip' and we present diagnostics to investigate its physics with ULLYSES and X-Shooter data. We discuss how the dip may provide important information on wind-driving physics, and how this is of key relevance towards finding a new gold-standard for OB star mass-loss rates. For B supergiants below the Fe IV to III bi-stability jump, the terminal velocity is found to be independent of Z and M, while the mass-loss rate still varies as {dot}M ∝Z^0.85. For O-type stars above the bi-stability jump we, find a terminal-velocity dependence of v_∞∝Z^0.19 and the Z-dependence of the mass-loss rate is found to be as shallow as {dot}M ∝Z^0.42, implying that to reproduce the 'heavy' black holes from LIGO/Virgo, the 'low Z' requirement becomes even more stringent than was previously anticipated.