The R136 star cluster dissected with Hubble Space Telescope/STIS

III. The most massive stars and their clumped winds^★

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
e-mail: s.a.brands@uva.nl
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D Leuven, Belgium
³ Department of Physics and Astronomy University of Sheffield, Sheffield, S3 7RH, UK
⁴ LMU München, Universitätssternwarte, Scheinerstr. 1, 81679 München, Germany
⁵ Department of Aerospace, Physics and Space Sciences, Florida Institute of Technology, 150 W. University Boulevard, Melbourne, FL 32901, USA
⁶ European Southern Observatory, Alonso de Córdova 3107 Vitacura, Santiago, Chile
⁷ Centro de Astrobiología, CSIC-INTA, Crtra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz (Madrid), Spain
⁸ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁹ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
¹⁰ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
¹¹ Armagh Observatory, College Hill, Armagh BT61 9DG, UK

Received: 24 November 2021
Accepted: 22 February 2022

Abstract

Context. The star cluster R136 inside the Large Magellanic Cloud hosts a rich population of massive stars, including the most massive stars known. The strong stellar winds of these very luminous stars impact their evolution and the surrounding environment. We currently lack detailed knowledge of the wind structure that is needed to quantify this impact.

Aims. Our goal is to observationally constrain the stellar and wind properties of the massive stars in R136, in particular the wind-structure parameters related to wind clumping.

Methods. We simultaneously analyse optical and ultraviolet spectroscopy of 53 O-type and three WNh-stars using the Fastwind model atmosphere code and a genetic algorithm. The models account for optically thick clumps and effects related to porosity and velocity-porosity, as well as a non-void interclump medium.

Results. We obtain stellar parameters, surface abundances, mass-loss rates, terminal velocities, and clumping characteristics and compare them to theoretical predictions and evolutionary models. The clumping properties include the density of the interclump medium and the velocity-porosity of the wind. For the first time, these characteristics are systematically measured for a wide range of effective temperatures and luminosities.

Conclusions. We confirm a cluster age of 1.0–2.5 Myr and derived an initial stellar mass of ≥250 M_⊙ for the most massive star in our sample, R136a1. The winds of our sample stars are highly clumped, with an average clumping factor of f_cl = 29 ± 15. We find tentative trends in the wind-structure parameters as a function of the mass-loss rate, suggesting that the winds of stars with higher mass-loss rates are less clumped. We compare several theoretical predictions to the observed mass-loss rates and terminal velocities and find that none satisfactorily reproduce both quantities. The prescription of Krtička & Kubát (2018) matches the observed mass-loss rates best.