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

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, Garching bei München 85748, Germany
² Astronomical Institute of Charles University, V Holešovičkách 2, Prague 18000, Czech Republic
³ Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, E-38205 La Laguna, Tenerife, Spain

^⋆ Corresponding author; jaroslav.merc@gmail.com

Received: 22 August 2024
Accepted: 21 November 2024

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 analyze 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 they 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, multifrequency spectral energy distribution, and published radial velocities and light curves, we constrained 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 such as symbiotic binaries, where the predominant mass transfer mode remains unclear.