X-Shooting ULLYSES: Massive stars at low metallicity. III. Terminal wind speeds of ULLYSES massive stars
Crowther, P. A.; Herrero, A.; Hamann, W. -R.; Puls, J.; de Koter, A.; Decin, L.; Ignace, R.; Sander, A. A. C.; Shenar, T.; Sana, H.; Bestenlehner, J. M.; Vink, J. S.; Najarro, F.; Brands, S. A.; Hawcroft, C.; Mahy, L.; Pauli, D.; ud-Doula, A.; Garcia, M.; Kubátová, B.; Oskinova, L.; St-Louis, N.; Sundqvist, J. O.; Erba, C.; David-Uraz, A.; Kee, N. D.; Lefever, R.; Moffat, A.; Prinja, R.
Belgium, United States, Netherlands, United Kingdom, Spain, Germany, Czech Republic, Canada
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∞, 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 (Teff), metallicity (Z), and surface escape velocity vesc.
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 Teff and vesc 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∞ ∝ Z0.22±0.03 when we compared our results to previous Galactic studies.
Conclusions: Our results suggest that Teff rather than vesc should be used as a straightforward empirical prediction of v∞ and that the observed Z dependence is steeper than suggested by earlier works.