X-Shooting ULLYSES: Massive stars at low metallicity III. Terminal wind speeds of ULLYSES massive stars

Context. The winds of massive stars have a significant impact on stellar evolution and on the surrounding medium. The maximum speed reached by these outflows, the terminal wind speed v_∞, is a global wind parameter and an essential input for models of stellar atmospheres and feedback. With the arrival of the ULLYSES programme, a legacy UV spectroscopic survey with the Hubble Space Telescope, we have the opportunity to quantify the wind speeds of massive stars at sub-solar metallicity (in the Large and Small Magellanic Clouds, 0.5 Z_☉ and 0.2 Z_☉, respectively) at an unprecedented scale. Aims. We empirically quantify the wind speeds of a large sample of OB stars, including supergiants, giants, and dwarfs at sub-solar metallicity. Using these measurements, we investigate trends of v_∞ with a number of fundamental stellar parameters, namely effective temperature (T_eff), metallicity (Z), and surface escape velocity v_esc. Methods. We empirically determined v_∞ for a sample of 149 OB stars in the Magellanic Clouds either by directly measuring the maximum velocity shift of the absorption component of the C IV λλ1548–1550 line profile, or by fitting synthetic spectra produced using the Sobolev with exact integration method. Stellar parameters were either collected from the literature, obtained using spectral-type calibrations, or predicted from evolutionary models. Results. We find strong trends of v_∞ with T_eff and v_esc when the wind is strong enough to cause a saturated P Cygni profile in C IV λλ1548–1550. We find evidence for a metallicity dependence on the terminal wind speed v_∞ ∝ Z^0.22±^0.03 when we compared our results to previous Galactic studies. Conclusions. Our results suggest that T_eff rather than v_esc should be used as a straightforward empirical prediction of v_∞ and that the observed Z dependence is steeper than suggested by earlier works.