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.; 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.
Bibliographical reference

Astronomy and Astrophysics

Advertised on:
8
2024
Number of authors
29
IAC number of authors
1
Citations
17
Refereed citations
9
Description
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.