Binarity at LOw Metallicity (BLOeM)

Enhanced multiplicity of early B-type dwarfs and giants at Z = 0.2 Z_⊙^⋆

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Royal Observatory of Belgium, Avenue Circulaire/Ringlaan 3, B-1180 Brussels, Belgium
⁴ The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 6997801, Israel
⁵ ESO - European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁶ Instituto de Astrofísica de Canarias, C. Vía Láctea, s/n, 38205 La Laguna, Santa Cruz de Tenerife, Spain
⁷ University of Wyoming, Physics & Astronomy Department, 1000 E. University Ave., Laramie, WY 82071, USA
⁸ Centro de Astrobiología (CSIC-INTA), Ctra. Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Spain
⁹ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 43, SE-221 00 Lund, Sweden
¹⁰ School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
¹¹ School of Mathematical and Physical Sciences, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK
¹² Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany
¹³ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁴ Villanova University, 800 E. Lancaster Ave., Villanova, PA 19085, USA
¹⁵ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁶ Centro de Astrobiología (CSIC-INTA), Campus ESAC, camino bajo del castillo s/n, 28 692 Villanueva de la Cañada, Spain
¹⁷ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
¹⁸ ARC Centre of Excellence for Gravitational-wave Discovery (OzGrav), Melbourne, Australia
¹⁹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
²⁰ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
²¹ Department of Astronomy & Steward Observatory, 933 N. Cherry Ave., Tucson, AZ 85721, USA
²² Observatório Nacional, R. Gen. José Cristino, 77 – Vasco da Gama, Rio de Janeiro (RJ) 20921-400, Brazil
²³ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
²⁴ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
²⁵ Lennard-Jones Laboratories, Keele University, Keele ST5 5BG, UK
²⁶ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, the Netherlands
²⁷ Armagh Observatory, College Hill, Armagh, BT61 9DG Northern Ireland, UK

^⋆⋆ Corresponding author: villasenor@mpia.de

Received: 26 November 2024
Accepted: 23 March 2025

Abstract

Early B-type stars with initial masses between 8 and 15 M_⊙ are frequently found in multiple systems, as is evidenced by multi-epoch spectroscopic campaigns in the Milky Way and the Large Magellanic Cloud (LMC). Previous studies have shown no strong metallicity dependence in the close-binary (a < 10 au) fraction or orbital-period distributions between the Milky Way’s solar metallicity (Z_⊙) and that of the LMC (Z = 0.5 Z_⊙). However, similar analyses for a large sample of massive stars in more metal-poor environments are still scarce. We focus on 309 early B-type stars (luminosity classes III-V) from the Binarity at LOw Metallicity (BLOeM) campaign, which targeted nearly 1000 massive stars in the Small Magellanic Cloud (SMC, Z = 0.2 Z_⊙) using VLT/FLAMES multi-epoch spectroscopy. By applying binary detection criteria consistent with previous works on Galactic and LMC samples, we identify 153 stars (91 SB1, 59 SB2, 3 SB3) exhibiting significant radial-velocity (RV) variations, resulting in an observed multiplicity fraction of f_mult^obs = 50 ± 3%. Using Monte Carlo simulations to account for observational biases, we infer an intrinsic close-binary fraction of f_mult = 80 ± 8%. This fraction reduces to ∼55% when increasing our RV threshold from 20 to 80 km s⁻¹; however, an independent Markov chain Monte Carlo analysis of the peak-to-peak RV distribution (ΔRV_max) confirms a high multiplicity fraction of f_mult = 79 ± 5%. These findings suggest a possible anti-correlation between metallicity and the fraction of close B-type binaries, with the SMC multiplicity fraction significantly exceeding previous measurements in the LMC and the Galaxy. The enhanced fraction of close binaries at SMC’s low metallicity may have broad implications for massive-star evolution in the early Universe. More frequent mass transfer and envelope stripping could boost the production of exotic transients, stripped supernovae, gravitational-wave progenitors, and sustained UV ionising flux, potentially affecting cosmic reionisation. Theoretical predictions of binary evolution under metal-poor conditions will provide a key test of our results.