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

Abstract

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<SUB>∞</SUB>, 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<SUB>⊙</SUB> and 0.2 Z<SUB>⊙</SUB>, respectively) at an unprecedented scale. <BR /> 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<SUB>∞</SUB> with a number of fundamental stellar parameters, namely effective temperature (T<SUB>eff</SUB>), metallicity (Z), and surface escape velocity v<SUB>esc</SUB>. <BR /> Methods: We empirically determined v<SUB>∞</SUB> 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. <BR /> Results: We find strong trends of v<SUB>∞</SUB> with T<SUB>eff</SUB> and v<SUB>esc</SUB> 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<SUB>∞</SUB> ∝ Z<SUP>0.22±0.03</SUP> when we compared our results to previous Galactic studies. <BR /> Conclusions: Our results suggest that T<SUB>eff</SUB> rather than v<SUB>esc</SUB> should be used as a straightforward empirical prediction of v<SUB>∞</SUB> and that the observed Z dependence is steeper than suggested by earlier works.