Insights into the planetary dynamics of HD 206893 with ALMA

Radial substructure in the form of rings and gaps has been shown to be ubiquitous among protoplanetary discs. This could be the case in exoKuiper belts as well, and evidence for this is emerging. In this paper we present ALMA observations of the debris/planetesimal disc surrounding HD 206893, a system that also hosts two massive companions at 2 and 11 au. Our observations reveal a disc extending from 30 to 180 au, split by a 27 au wide gap centred at 74 au, and no dust surrounding the reddened brown dwarf (BD) at 11 au. The gap width suggests the presence of a 0.9 M$_\mathrm{Jup}$ planet at 74 au, which would be the third companion in this system. Using previous astrometry of the BD, combined with our derived disc orientation as a prior, we were able to better constrain its orbit finding it is likely eccentric ($0.14^{+0.05}_{-0.04}$). For the innermost companion, we used RV, proper motion anomaly and stability considerations to show its mass and semi-major axis are likely in the range 4-100 M$_\mathrm{Jup}$ and 1.4-4.5 au. These three companions will interact on secular timescales and perturb the orbits of planetesimals, stirring the disc and potentially truncating it to its current extent via secular resonances. Finally, the presence of a gap in this system adds to the growing evidence that gaps could be common in wide exoKuiper belts. Out of 6 wide debris discs observed with ALMA with enough resolution, 4-5 show radial substructure in the form of gaps.