Binarity at LOw Metallicity (BLOeM)

A spectroscopic VLT monitoring survey of massive stars in the SMC^★

¹ The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 6997801 Israel
² ESO – European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
³ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ Department of Physics & Astronomy, Hounsfield Road, University of Sheffield, Sheffield S3 7RH, UK
⁵ Instituto de Astrofísica de Canarias, C. Vía Láctea, s/n, 38205 La Laguna, Santa Cruz de Tenerife, Spain
⁶ Universidad de La Laguna, Dpto. Astrofísica, Av. Astrofśico Francisco Sánchez, 38206 La Laguna, Santa Cruz de Tenerife, Spain
⁷ Escola de Ciências e Tecnologia, Universidade Federal do Rio Grande do Norte, Natal, RN 59072-970, Brazil
⁸ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
⁹ School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
¹⁰ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
¹¹ Universität Heidelberg, Department of Physics and Astronomy, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany
¹² Royal Observatory of Belgium, Avenue Circulaire/Ringlaan 3, 1180 Brussels, Belgium
¹³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁴ Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany
¹⁵ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁶ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
¹⁷ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí Franquès 1, 08028 Barcelona, Spain
¹⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
²⁰ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
²¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²² Department of Astronomy and Physics, Saint Mary’s University, 923 Robie Street, Halifax B3H 3C3, Canada
²³ Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
²⁴ Center for Computational Astrophysics, Division of Science, National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
²⁵ Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 298, CZ-251 65 Ondřejov, Czech Republic
²⁶ Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario, M5S 3H4, Canada
²⁷ Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dunsink Lane, Dublin 15, Ireland
²⁸ Centro de Astrobiología (CSIC-INTA), campus ESAC, camino bajo del castillo s/n, 28 692 Villanueva de la Cañada, Spain
²⁹ School of Physics and Astronomy, Monash University, Clayton VIC 3800, Australia
³⁰ ARC Centre of Excellence for Gravitational-wave Discovery (OzGrav), Melbourne, Australia
³¹ IAASARS, National Observatory of Athens, 15236 Penteli, Greece
³² Institute of Astrophysics, FORTH, 71110 Heraklion, Greece
³³ Centro de Astrobiología (CSIC-INTA), Ctra. Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Spain
³⁴ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
³⁵ McWilliams Center for Cosmology & Astrophysics, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
³⁶ Lund Observatory, Division of Astrophysics, Department of Physics, Lund University, Box 43, 221 00, Lund, Sweden
³⁷ Department of Astronomy & Steward Observatory, 933 N. Cherry Ave., Tucson, AZ 85721, USA
³⁸ Observatório Nacional, R. Gen. José Cristino, 77 – Vasco da Gama, Rio de Janeiro, RJ 20921-400, Brazil
³⁹ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87–100 Torun, Poland
⁴⁰ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 7610001, Israel
⁴¹ Lennard-Jones Laboratories, Keele University, ST5 5BG, UK
⁴² Center for Computational Astrophysics, Flatiron Institute, New York, NY 10010, USA
⁴³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴⁴ Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland, UK

^★★ Corresponding author; tshenar@tau.ac.il

Received: 19 July 2024
Accepted: 27 August 2024

Abstract

Surveys in the Milky Way and Large Magellanic Cloud have revealed that the majority of massive stars will interact with companions during their lives. However, knowledge of the binary properties of massive stars at low metallicity, and therefore in conditions approaching those of the Early Universe, remain sparse. We present the Binarity at LOw Metallicity (BLOeM) campaign, an ESO large programme designed to obtain 25 epochs of spectroscopy for 929 massive stars in the Small Magellanic Cloud, allowing us to probe multiplicity in the lowest-metallicity conditions to date (Z = 0.2 Z_⊙). BLOeM will provide (i) the binary fraction, (ii) the orbital configurations of systems with periods of P ≲ 3 yr, (iii) dormant black-hole binary candidates (OB+BH), and (iv) a legacy database of physical parameters of massive stars at low metallicity. Main sequence (OB-type) and evolved (OBAF-type) massive stars are observed with the LR02 setup of the GIRAFFE instrument of the Very Large Telescope (3960–4570 Å resolving power R = 6200; typical signal-to-noise ratio(S/N) ≈70–100). This paper utilises the first nine epochs obtained over a three-month time period. We describe the survey and data reduction, perform a spectral classification of the stacked spectra, and construct a Hertzsprung-Russell diagram of the sample via spectral-type and photometric calibrations. Our detailed classification reveals that the sample covers spectral types from O4 to F5, spanning the effective temperature and luminosity ranges 6.5 ≲ T_eff/kK ≲ 45 and 3.7 < log L/L_⊙ < 6.1 and initial masses of 8 ≲ M_ini ≲ 80 M_⊙. The sample comprises 159 O-type stars, 331 early B-type (B0–3) dwarfs and giants (luminosity classes V–III), 303 early B-type supergiants (II–I), and 136 late-type BAF supergiants. At least 82 stars are OBe stars: 20 O-type and 62 B-type (13% and 11% of the respective samples). In addition, the sample includes 4 high-mass X-ray binaries, 3 stars resembling luminous blue variables, 2 bloated stripped-star candidates, 2 candidate magnetic stars, and 74 eclipsing binaries.