The GAPS programme at TNG LXXI. HD 128717 B/Gaia-6 B: a long-period eccentric low-mass brown dwarf from astrometry and radial velocities

TL;DR

This work confirms Gaia-6 B as a long-period, high-eccentricity brown dwarf orbiting HD 128717 by combining high-precision HARPS-N RVs with absolute astrometry from Hipparcos and Gaia through a PMa+RV joint model, yielding $P_{ m b}=9.37$ yr, $M_{ m b}=19.8\pm0.5\rm \,M_J$, $e_{ m b}=0.850\pm0.002$, and $i_{ m b}=130.3^{\circ}\,+/-1.9$. The Gaia DR3 solution is shown to be degenerate due to the short DR3 timespan and the long period/high eccentricity of the companion, a result supported by Gaia astrometric simulations; Gaia DR4 is expected to better recover the true orbit. No evidence for additional inner or outer companions is found from RV residuals and direct imaging, with imaging constraining distant companions to be below ~11–47 $M_J$ at separations beyond ~60 au for an age around 1.4–1.9 Gyr. The findings place Gaia-6 B in the brown dwarf regime, discuss its formation context given host metallicity, and highlight the value of RV follow-up to validate Gaia astrometric candidates and refine orbital parameters.

Abstract

The transition regime between giant planets (GPs) and brown dwarfs (BDs) is still an open subject of study in exoplanetary science. A complete understanding of the population of long-period GPs and BDs would be pivotal in understanding this topic, but the number of such objects with precisely measured orbital and physical parameters is still small. Moreover, their dynamical influence on smaller companions in inner orbits is still unclear. Within the GAPS programme, we aim to confirm and characterize sub-stellar companion candidates from Gaia DR3, and to study the potential presence of additional lower-mass planets in their systems. We present the results of an intensive high-precision RV monitoring of HD 128717, which hosts the astrometric candidate Gaia-ASOI-009. We used the HARPS-N spectrograph at TNG to collect a high-cadence RV time series of the target. We used 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-6 B: from the combined RV+PMa fit, we confirm that it is a high-eccentricity low-mass brown dwarf with $P_\text{B} = 9.37^{+0.06}_{-0.05}$ yr, $M_\text{B} = 19.8 \pm 0.5$ $M_\text{J}$, $e_\text{B}=0.85$, $i_\text{B} = 130^{\circ}$. 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-6 B 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.

Figures (20)

• Figure 1: Upper panel: HARPS-N RV time series of HD 128717 (blue points), compared with the predicted RV signals from the Gaia orbital solution (red dashed line). Lower panel: HARPS-N $\log R'_\text{HK}$ time series.
• Figure 2: Stellar spectral energy distribution (SED). The broad-band measurements from the Tycho, 2MASS and WISE magnitudes are shown in red and the corresponding theoretical values with blue circles. The unaveraged best-fit model is displayed with a black solid line.
• Figure 3: GLS periodograms of the spectroscopic time series. Upper panel: RV; Lower panel:$\log R'_\text{HK}$.
• Figure 4: RV-activity correlation: RV time series as a function of $\log R'_\text{HK}$.
• Figure 5: RV-activity correlation: RV residuals time series as a function of $\log R'_\text{HK}$.