Unequivocal detection of the tidal deformation of a red giant in a binary system via interferometry

TL;DR

This study presents the first direct interferometric detection of tidal deformation in a red giant within a binary system, providing new insights into mass transfer processes and binary evolution.

Contribution

The paper demonstrates that interferometry can resolve Roche-lobe-filling giants and directly observe tidal deformation, advancing understanding of binary mass transfer mechanisms.

Findings

Red giant in HD 352 is tidally deformed and fills its Roche lobe.

Interferometric observations match Roche-lobe shape rather than symmetric models.

HD 352 is likely approaching the common envelope phase.

Abstract

While mass transfer in binary systems is a crucial aspect of binary evolution models, it remains far from understood. HD 352 is a spectroscopic binary exhibiting ellipsoidal variability, likely due to a tidally deformed giant donor filling its Roche lobe and transferring matter to a faint companion. Here, we analyse VLTI/PIONIER interferometric observations of the system, obtained between 2010 to 2020. We demonstrate that observations near the system's quadrature cannot be explained by simple symmetric disk models, but are consistent with the shape of a Roche-lobe-filling star. We think that this is the first case of tidal deformation of a red giant being observed directly, thanks to the interferometric technique. By combining our interferometric modeling results with the analysis of the optical spectrum, multi-frequency spectral energy distribution, and published radial velocities and light curves, we constrain the system parameters and show that HD 352 will likely soon enter the common envelope phase, although we cannot reject the hypothesis that it is undergoing stable mass transfer against theoretical predictions. This has important consequences for modeling a large class of binary systems. Additionally, our observations confirm that Roche-lobe-filling giants can be resolved with interferometry under favorable conditions. Such observations may help resolve the mass transfer dichotomy in systems like symbiotic binaries, where the predominant mass transfer mode remains unclear.