Hawcroft, C. ; Sana, H. ; Mahy, L. ; Sundqvist, J. O. ; de Koter, A. ; Crowther, P. A. ; Bestenlehner, J. M. ; Brands, S. A. ; David-Uraz, A. ; Decin, L. ; Erba, C. ; Garcia, M. ; Hamann, W. -R. ; Herrero, A. ; Ignace, R. ; Kee, N. D. ; Kubátová, B. ; Lefever, R. ; Moffat, A. ; Najarro, F. ; Oskinova, L. ; Pauli, D. ; Prinja, R. ; Puls, J. ; Sander, A. A. C. ; Shenar, T. ; St-Louis, N. ; ud-Doula, A. ; Vink, J. S.

published in Astronomy & Astrophysics, 688 issue A105, pp. 19 (2024)

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.

DOI: 10.1051/0004-6361/202245588