Interferometry of Massive Stars: Multiplicity, Magnetism, and Stellar Winds

TL;DR

Interferometry of Massive Stars shows that long-baseline techniques deliver (sub)au-scale measurements crucial for characterizing multiplicity, magnetism, and winds in massive stars. By integrating SMASH+ and VLTI/GRAVITY results, the paper demonstrates that multiplicity is nearly universal among massive stars, with hierarchical triple architectures and strong dynamical interactions shaping their evolution and endpoints. Magnetic phenomena appear linked to merger histories, while wind-formation regions in Wolf–Rayet stars become accessible to direct imaging, offering new constraints on wind physics. Looking ahead, kilometer-baseline interferometers promise to resolve inner binaries and map wind structures with unprecedented precision, refining models of binary evolution and the progenitors of compact objects and gravitational-wave sources.

Abstract

After decades of efforts, optical long-baseline interferometry has become a mainstream observational technique in terms of operation robustness and user friendliness. Interferometry has opened a new observational window, enabling (sub)au-scale resolution of massive stars and direct measurements of orbital parameters, wind structures, and magnetic phenomena. This paper reviews recent advances in interferometric studies of massive stars, focusing on multiplicity, magnetism, and stellar winds.

Figures (3)

• Figure 1: (a) Binary detection probability of an interferometric campaign towards galactic early B-type stars (average distance 0.5 kpc); adapted from Frost2025. (b) and (c) Relative astrometric orbits of $\delta$ Cir and MY Ser (adapted from Sana+, in prep.)
• Figure 2: (a) Inner vs. outer (projected) separation diagram of triple systems in the SMaSH+ sample. (b) Resolved $K$-band VLTI/GRAVITY spectra of the magnetic binary system HD 148937. Panels (a) and (b) are adapted from Bordier2026 and Frost2024.
• Figure 3: Resolving wind line emission regions in WR 78. (a) Interferometric observables of VLTI/Gravity observations. Bottom: normalized flux, revealing the presence of strong He I and He II lines. Top: closure phase spectra along two baselines, revealing a drop at the location of the He I 2.11-2.16$\mu$m lines, but not at that of the He II 2.19$\mu$m line, suggesting that the latter is unresolved while the former are (at least partially) resolved. (b) Predicted line formation strengths of various lines (see legend) in the wind of the WR78, computed with the PoWR atmospheric model powr2015powr2002powr2003. The approximate inner working angle limit of the Gravity observation is indicated by the vertical dashed line. (c) Up-to-scale sketch of the relative size of the peak emission radii of the different lines in panel (b). Panels (a) and (b) are adapted from Deshmukh2024.