X-Shooting ULLYSES: Massive stars at low metallicity

III. Terminal wind speeds of ULLYSES massive stars

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001, Leuven, Belgium
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
e-mail: chawcroft@stsci.edu
³ Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium
⁴ Astronomical Institute Anton Pannekoek, Amsterdam University, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁵ Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK
⁶ Department of Physics and Astronomy, Howard University, Washington, DC 20059, USA
⁷ Center for Research and Exploration in Space Science and Technology, and X-ray Astrophysics Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA
⁸ Department of Physics & Astronomy, East Tennessee State University, Johnson City, TN 37614, USA
⁹ Centro de Astrobiología, CSIC-INTA. Crtra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁰ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
¹¹ Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
¹² Departamento de Astrofisica, Universidad de La Laguna, 38205, La Laguna, Tenerife, Spain
¹³ National Solar Observatory, 22 Ohi’a Ku St, Makawao, HI 96768, USA
¹⁴ Astronomický ústav, Akademie věd České republiky, CZ-251 65 Ondřejov, Czech Republic
¹⁵ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
¹⁶ Département de Physique and Centre de Recherche en Astrophysique du Québec (CRAQ) Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada
¹⁷ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁸ LMU München, Universitätssternwarte, Scheinerstr. 1, 81679 München, Germany
¹⁹ Dept. of Physics, Penn State Scranton, 120 Ridge View Drive, Dunmore, PA 18512, USA
²⁰ Armagh Observatory and Planetarium, College Hill, BT61 9DG Armagh, Northern Ireland, UK

Received: 30 November 2022
Accepted: 13 February 2023

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 (T_eff), metallicity (Z), and surface escape velocity v_esc.

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 T_eff and v_esc 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_∞ ∝ Z^0.22±0.03 when we compared our results to previous Galactic studies.

Conclusions. Our results suggest that T_eff rather than v_esc should be used as a straightforward empirical prediction of v_∞ and that the observed Z dependence is steeper than suggested by earlier works.