Modelling a Transiting Circumbinary Disc in the HD98800 System

Abstract

We present synthetic optical light curves of the hierarchical HD98800 quadruple system over a decade-long period when the circumbinary disc encircling the system's B binary is expected to eclipse the light from the A binary. We produce and compare light curves of this transit event using hydrodynamical models with different values of the disc's gas mass, dust mass, and $\alpha$-viscosity to determine the observable effect of each parameter. These comparisons provide insight that could aid in the analysis of observational data from the system when the real transit occurs and provide recommendations for how such observations should be made. We find that a higher dust mass or higher value of $\alpha$ correspond to a longer transit, with the gas mass having a more minor effect on the overall shape and duration of the transit. A higher $\alpha$ has an observable effect on the viscous spreading at the outer edge of the disc, though is countered through truncation by the outer binary. It is also shown that long-term interactions between the outer binary and disc can excite spiral arms in the disc, which introduce observable asymmetries to the light curve. Our models suggest that the transit should have begun at the time of writing, but no dimming has yet been observed. It is likely that the disc has a smaller radial extent than our models, due to a lower viscosity than can be simulated with SPH. The transit is expected to last 8-11 years, ending in late 2034 at the latest.