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.
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.
This paper in ADS