Peering through the disc of HD 98800 BaBb. Precise timing predictions for the HD 98800 AaAb occultation

Abstract

HD 98800 is a young hierarchical quadruple system composed of the tight binaries AaAb and BaBb on a wide, highly inclined outer orbit. The B subsystem hosts a circumbinary disc in a polar configuration, and the geometry of the system offers a rare opportunity to observe the passage of the disc around BaBb in front of AaAb. We aim to update the overall orbital solution to offer precise time windows of the main occultation features by combining the orbit with the state-of-the-art knowledge of the disc's structure. We combine new and published radial-velocity measurements and multi-wavelength astrometry for the outer AB orbit and both inner subsystems, AaAb and BaBb, in a joint orbital fit. The revised solution is consistent with previous dynamical masses and improved the outer orbit, reducing the uncertainties in the period and periastron epoch by about a factor of two. It also narrows the predicted crossing-phase windows to 5--15 days at the 1σ level, improving the timing of ingress, egress, and the first cavity-crossing predictions. These results provide a more accurate timing framework for future observations of the occultation, although the predicted epochs remain model-dependent because of uncertainties in the disc structure.