Investigating 39 Galactic Wolf-Rayet stars with VLTI/GRAVITY

Uncovering a long-period binary desert^⋆

¹ Institute of Astronomy, KU Leuven, Celestijnlaan 200D, 3001 Leuven, Belgium
² Leuven Gravity Institute, KU Leuven, Celestijnenlaan 200D, box 2415 3001 Leuven, Belgium
³ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁴ I. Physikalisches Institut, Universität zu Kön, Zülpicher Str. 77, 50937 Köln, Germany
⁵ Argelander Institut für Astronomie, Auf dem Hügel 71, DE-53121 Bonn, Germany
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, DE-53121 Bonn, Germany
⁷ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles CP 226, Boulevard du Triomphe, 1050 Brussels, Belgium
⁸ European Southern Observatory, Santiago, Chile
⁹ Groupe d’Astrophysique des Hautes Energies, STAR, Université de Liège, Quartier Agora (B5c, Institut d’Astrophysique et de Géophysique), Allée du 6 Août 19c, B-4000 Sart Tilman, Liége, Belgium
¹⁰ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹¹ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
¹² Royal Observatory of Belgium, B-1180 Brussels, Belgium
¹³ Departamento de Astrofísica, Centro de Astrobiología, (CSIC-INTA), Ctra. Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁴ The School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 6997801 Israel
¹⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹⁶ Independent Researcher, Rehovot, Israel
¹⁷ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot, 76100 Israel

^⋆⋆ Corresponding author; kunalprashant.deshmukh@kuleuven.be

Received: 23 September 2024
Accepted: 4 November 2024

Abstract

Context. Wolf-Rayet stars (WRs) represent one of the final evolutionary stages of massive stars and are thought to be the immediate progenitors of stellar-mass black holes. Their multiplicity characteristics form an important anchor point in single and binary population models for predicting gravitational-wave progenitors. Recent spectroscopic campaigns have suggested incompatible multiplicity fractions and period distributions for N- and C-rich Galactic WRs (WNs and WCs) at both short and long orbital periods, in contradiction with evolutionary model predictions.

Aims. In this work, we employed long-baseline infrared interferometry to investigate the multiplicity of WRs at long periods and explored the nature of their companions. We present a magnitude-limited (K <  9; V <  14) survey of 39 Galactic WRs, including 11 WN, 15 WC, and 13 H-rich WN (WNh) stars.

Methods. We used the K-band instrument GRAVITY at the Very Large Telescope Interferometer (VLTI) in Chile. The sensitivity of GRAVITY at spatial scales of ∼1 to 200 milliarcseconds and flux contrast of 1% allowed an exploration of periods in the range 10² − 10⁵ d and companions down to ∼5 M_⊙. We carried out a companion search for all our targets, with the aim of either finding wide companions or calculating detection limits. We also explored the rich GRAVITY dataset beyond a multiplicity search to look for other interesting properties of the WR sample.

Results. We detected wide companions with VLTI/GRAVITY for only four stars in our sample: WR 48, WR 89, WR 93, and WR 115. Combining our results with spectroscopic studies, we arrived at observed multiplicity fractions of f_obs^WN = 0.55 ± 0.15, f_obs^WC = 0.40 ± 0.13, and f_obs^WNh = 0.23 ± 0.12. The multiplicity fractions and period distributions of WNs and WCs are consistent in our sample. For single WRs, we placed upper limits on the mass of potential companions down to ∼5 M_⊙ for WNs and WCs, and ∼7 M_⊙ for WNh stars. In addition, we also found other features in the GRAVITY dataset, such as (i) a diffuse extended component contributing significantly to the K-band flux in over half the WR sample; (ii) five known spectroscopic binaries resolved in differential phase data, which constitutes an alternative detection method for close binaries; and (iii) spatially resolved winds in four stars: WR 16, WR 31a, WR 78, and WR 110.

Conclusions. Our survey reveals a lack of intermediate- (a few hundred days) and long- (a few years to decades) period WR systems. The 200d peak in the period distributions of WR+OB and BH+OB binaries predicted by Case B mass-transfer binary evolution models is not seen in our data. The rich companionship of their O-type progenitors in this separation range suggests that the WR progenitor stars expand and interact with their companions, most likely through unstable mass transfer, resulting in either a short-period system or a merger.