Full orbital solutions in pre-main sequence high-order multiple systems: GG Tau Ab and UX Tau B

High-order multiple (triple and beyond) systems are relatively common. Their interaction with circumstellar and circumbinary material can have a large impact on the formation and evolution of planetary systems and depends on their orbital properties. GG\, Tau and UX\, Tau are two pre-main sequence high-order multiple systems in which the tightest pair has a projected separation of 5--20\, au. Characterizing precisely their orbits is crucial to establish their long-term stability, to predict the dynamics and evolution of circumstellar matter, and to evaluate the potential for planet formation in such systems. We combine existing astrometric measurements with previously unpublished high-resolution observations of the GG\, Tau\, Ab and UX\, Tau\, B pairs and perform Keplerian orbital fits. For GG\, Tau\, Ab the data presented here represent the first detection of orbital motion. For both systems they yield dramatic increases in orbital coverage (60\% and 100\% for UX\, Tau\, B and GG\, Tau\, Ab, for orbital periods of 32 and 8\, yr, respectively) and allow us to obtain well-constrained orbital fits, including dynamical masses with 10\% and 7\% random and systematic uncertainties. We find that both GG\, Tau\, A and UX\, Tau\, A--B likely form stable hierarchical systems, although one possible deprojection solution for GG\, Tau is strongly misaligned and could experience von Zeipel-Lidov-Kozai oscillations. We further find that the UX\, Tau\, B orbit is much more eccentric than the GG\, Tau\, Ab one, possibly explaining the lack of circumstellar material in the former. The newly-determined orbits revive the question of the dynamical fate of gas and dust in these two hierarchical systems and should spur new dedicated simulations to assess the long-term evolution of the systems and the dynamical perturbations imposed by the close binaries they host.