Binarity at LOw Metallicity (BLOeM)

Multiplicity properties of Oe and Be stars^★

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
² ESO – European Southern Observatory, Karl-SchwarzschildStrasse 2, 85748 Garching bei München, Germany
³ The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 6997801, Israel
⁴ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Leuven Gravity Institute, KU Leuven, Celestijnenlaan 200D box 2415, 3001 Leuven, Belgium
⁶ Royal Observatory of Belgium, Avenue Circulaire/Ringlaan 3, 1180 Brussels, Belgium
⁷ Department of Physics & Astronomy, Hounsfield Road, University of Sheffield, Sheffield S3 7RH, UK
⁸ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁹ Instituto de Astrofísica de Canarias, C. Vía Láctea, s/n, 38205 La Laguna, Santa Cruz de Tenerife, Spain
¹⁰ Universidad de La Laguna, Dpto. Astrofísica, Av. Astrofśico Francisco Sánchez, 38206 La Laguna, Santa Cruz de Tenerife, Spain
¹¹ Centro de Astrobiología (CSIC-INTA), Ctra. Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Spain
¹² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹³ School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
¹⁴ Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany
¹⁵ Department of Astrophysics and Planetary Science, Villanova University, 800 E Lancaster Ave., PA 19085, USA
¹⁶ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁷ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
¹⁸ Astrophysics Group, Department of Physics, University of Surrey, Guildford GU2 7XH, UK
¹⁹ Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
²⁰ Center for Computational Astrophysics, Division of Science, National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
²¹ Astronomical Institute, Academy of Sciences of the Czech Republic, Fričova 298, 251 65 Ondřejov, Czech Republic
²² School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
²³ ARC Centre of Excellence for Gravitational-wave Discovery (OzGrav), Melbourne, Australia
²⁴ IAASARS, National Observatory of Athens, 15236 Penteli, Greece
²⁵ Institute of Astrophysics, FORTH, GR-71110, Heraklion, Greece
²⁶ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
²⁷ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
²⁸ Observatório Nacional, R. Gen. José Cristino, 77 – Vasco da Gama, Rio de Janeiro, RJ 20921-400, Brazil
²⁹ Department of Astronomy & Steward Observatory, 933 N. Cherry Ave., Tucson, AZ 85721, USA
³⁰ Heidelberger Institut für Theoretische Studien, SchlossWolfsbrunnenweg 35, 69118 Heidelberg, Germany
³¹ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
³² Lennard-Jones Laboratories, Keele University, ST5 5BG, UK
³³ Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK

^★★ Corresponding author: j.bodensteiner@uva.nl

Received: 15 October 2024
Accepted: 10 January 2025

Abstract

Context. Rapidly rotating classical OBe stars have been proposed as the products of binary interactions, and the fraction of Be stars with compact companions implies that at least some are. However, to constrain the interaction physics spinning up the OBe stars, a large sample of homogeneously analyzed OBe stars with well-determined binary characteristics and orbital parameters are required.

Aims. We investigated the multiplicity properties of a sample of 18 Oe, 62 Be, and two Of?p stars observed within the BLOeM survey in the Small Magellanic Cloud. We analyzed the first nine epochs of spectroscopic observations obtained over approximately three months in 2023.

Methods. Radial velocities (RVs) of all stars were measured using cross-correlation based on different sets of absorption and emission lines. Applying commonly used binarity criteria, we classified objects as binaries, binary candidates, and apparently single (RV stable) objects. We further inspected the spectra for double-lined spectroscopic binaries and cross-matched with catalogs of X-ray sources and photometric binaries.

Results. We classify 14 OBe stars as binaries, and an additional 11 as binary candidates. The two Of?p stars are apparently single. We find two more objects that are most likely currently interacting binaries. Without those, the observed binary fraction for the remaining OBe sample of 78 stars is f_obs+cand^OBe = 0.18 ± 0.04 (f_obs+cand^OBe = 0.32±0.05 including candidates). This binary fraction is less than half of that measured for OB stars in BLOeM. Combined with the lower fraction of SB2s, this suggests that OBe stars do indeed have fundamentally different present-day binary properties than OB stars. We find no evidence for OBe binaries with massive compact companions, in contrast to expectations from binary population synthesis.

Conclusions. Our results support the binary scenario as an important formation channel for OBe stars, as post-interaction binaries may have been disrupted or the stripped companions of OBe stars are harder to detect. Further observations are required to characterize the detected binaries, their orbital parameters, and the nature of their companions.