HD 128717 HARPS-N observations

We used Markov chain Monte Carlo (MCMC) analyses to refine the Gaia DR3 orbital solution of the companion and, finally, performed a combined model of RV and proper motion anomaly (PMa) to derive the complete 3-D orbit of the companion. We also ran a suite of numerical simulations to confirm our results. We confirm the sub-stellar nature of Gaia-ASOI-009, i.e. Gaia-6B: from the combined RV+PMa fit, we confirm that it is a high-eccentricity low-mass brown dwarf with P=9.37^+0.06^_-0.05_yr, M=19.8+/-0.5M_J_, e=0.85, i=130deg.

The derived orbital solution differs significantly from the one published in Gaia DR3. Through a series of dedicated simulations, we demonstrate that this discrepancy arises from a degeneracy in the Gaia DR3 astrometric solution. Specifically, the combination of Gaia-6B long orbital period and high eccentricity, both poorly constrained by the limited timespan of DR3, led to an incorrect solution characterized by a shorter period and lower eccentricity.

Finally, we found no evidence of other companions in the system, neither inner nor outer, and thus the origin of the high-eccentricity of Gaia-6B remains unclear.