Configuration of the ξ Tau system constrained by multi-technique observations

Context.ξ Tau is one of the most compact multiple stellar systems, which is sufficiently close (67 pc) to be constrained by all kinds of observations. Aims. To better constrain the current configuration of ξ Tau and to study its temporal evolution, we aim to utilize new observational data: (i) publicly available photometry from TESS and astrometry from WDS; and (ii) our own photometry from the MOST spacecraft and spectroscopy from the CTIO observatory. Methods. As a prerequisite, we accurately described the CTIO/CHIRON echelle instrument so that interpolation of the blaze function over the Balmer lines (H α , H β ) would allow us to rectify the wings with minimal systematics. We then used our previous model of ξ Tau and constrained it by the new data. At the same time, we improved our dynamical model by accounting not only for mutual, N-body perturbations, but also for relativistic effects, oblateness, and tides. We optimized grids of models, showing χ 2 contributions from 11 different types of observations (astrometry, radial velocities, transit-timing variations, light curves, interferometric visibility, closure phase, etc.). This allowed us to find the global, best-fit solution, keeping track of possible ‘tension’ among the different data. Results. Given the hierarchical architecture of ξ Tau, ((Aa+Ab)+B)+C, we detected the orbital evolution on all timescales. Oscillations of periods, P , occur on the shortest orbital timescales ( P 1 , P 2 ); the variation of eccentricity, e 1 , is from 0 to 0.008, and that of e 2 is from 0.202 to 0.207. Oscillations of projected i , Ω are coupled and occur on the secular timescale of about 7000 d. The inclination, i 1 , of the inner, eclipsing pair (Aa+Ab) changes from 86.1° to 87.1°, which is clearly manifested in eclipse depths. There is also a long-term trend due to the outer orbit ( P 3 ≐ 18900 d), with a perihelion passage (a ‘bump’) of component C, which is manifested in radial velocities. The mutual inclinations among the three orbital planes, ≃0.5° and 71°, are very different. Long-term stability is ensured by suppressing Kozai oscillations due to the fast precession rate ω ˙. The best model requires tidal dissipation in the inner binary (with the time lag of ∼100 s) and five components, where component C is a binary (Ca+Cb). Conclusions. Although the masses of the three components (2.27, 2.15, 3.78 M ⊙ ) are now constrained to within 1%, the suspected binary (Ca+Cb), offset by 600 mas, should be better characterized. A key question remains whether this bright stellar system contains additional dwarf or exoplanetary components with low masses. Continuing monitoring of ξ Tau is highly desirable.