How does the disk of a Be star behave when eclipsed by an sdOB companion?

In the heart of the Carina constellation, astronomers have discovered the first eclipsing binary system featuring a classical Be star dancing with an sdOB companion. Thanks to TESS data and multicolour observations gathered by a network of professionals and amateurs, the team has tracked the system’s brightness and colour changes along its orbit. Stellar eclipses, where the sdOB passes in front of or behind the Be, are well reproduced by radiative transfer models that include the star, its circumstellar disk, and the small companion. Yet two details escape the current explanation: during the first attenuation phase the observed colour becomes redder, while models predict a bluish shift; moreover, around the second eclipse the models show no light drop, although the data display a clear signal. These mismatches reveal that the interaction between the Be star’s gaseous disk and the companion’s gravity is more complex than previously thought. The disk may not be uniform, harbouring dense clumps or waves that alter opacity depending on viewing direction. Alternatively, the sdOB could disturb the disk material, lifting or heating it unevenly. Studying V658 Car thus offers a unique window into how stellar disks respond to binary forces, with implications for planet formation around massive stars and the evolution of unusual binary systems. Future high‑resolution spectroscopy and optical interferometry will map the disk’s density and temperature at different orbital phases, testing hypotheses of non‑uniform structures or mass transfer from the Be to the sdOB. These results will also refine models for other Be binaries, helping astronomers separate intrinsic disk variations from geometric eclipse effects.

The V658 Car system serves as a natural laboratory to study the physics of circumstellar disks under the influence of a hot, small sdOB companion. Be stars are characterised by a Keplerian free‑gas disk, produced by rapid rotation near the critical speed and sustained by turbulent viscosity or magnetic fields. In a binary setting, the companion exerts tidal forces that can truncate the disk to roughly one‑third of the orbital separation, depending on the mass ratio and eccentricity. Moreover, the intense ultraviolet radiation of the sdOB can ionise the disk’s surface layers, altering its opacity and causing selective heating that appears as colour variations not predicted by standard isotropic, static radiative transfer models. Observations show a reddening during the first attenuation phase, suggesting an increase in dust or larger grain sizes along the line of sight, possibly due to density‑wave spirals induced by the companion’s tidal potential. The lack of predicted attenuation around the second eclipse implies that, in that geometric configuration, the disk may be more transparent—perhaps because the line of sight passes through a low‑density region or because the disk has been partially emptied by mass streams toward the companion. Modelling these effects requires 3D hydrodynamic codes that include viscosity, irradiation, and possibly magnetic fields, together with a non‑local radiation treatment. Recent simulations show that warped or eccentric disks can reproduce exactly the observed reddening and missing eclipse pattern. Further spectroscopic analysis of Hα and metal emission lines will allow measurement of gas velocities and identification of any mass flows toward the sdOB. Finally, the insights gained from V658 Car can be transferred to other Be binaries, improving our ability to interpret the photometric and spectroscopic variability of these complex systems.