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

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. search by orcid ; Kubátová, B. ; Lefever, R. ; Moffat, A. ; Najarro, F. ; Oskinova, L. ; Pauli, D. ; Prinja, R. ; Puls, J. ; Sander, A. A. C. search by orcid ; Shenar, T. ; St-Louis, N. ; ud-Doula, A. ; Vink, J. S.

The winds of massive stars have an impact on stellar evolution and on the surrounding medium. The maximum speed reached by these outflows, the terminal wind speed, 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 HST, we have the opportunity to quantify the wind speeds of massive stars at sub-solar metallicity (in the Large and Small Magellanic Clouds, 0.5Z and 0.2Z) at an unprecedented scale. 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 terminal wind speed with a number of fundamental stellar parameters, namely effective temperature, metallicity, and surface escape velocity. We empirically determined the terminal wind speed 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 Civ 1548-1550 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. We find strong trends of terminal wind speed with effective temperature and surface escape speed when the wind is strong enough to cause a saturated P Cygni profile in Civ 1548-1550. We find evidence for a metallicity dependence on the terminal wind speed proportional to Z^0.22+-0.03 when we compared our results to previous Galactic studies. Our results suggest that effective temperature rather than surface escape speed should be used as a straightforward empirical prediction of terminal wind speed and that the observed metallicity dependence is steeper than suggested by earlier works.

Preprint (ULLYSES-III-Hawcroft+2023.pdf, 2.6MB)

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