J/A+A/707/A67 HD 128717 HARPS-N observations (Pinamonti+, 2026)
The GAPS programme at TNG. LXX.
HD128717 B/Gaia-6 B: A long-period eccentric low-mass brown dwarf from
astrometry and radial velocities.
Pinamonti M., Sozzetti A., Barbato D., Desidera S., Biazzo K., Bonomo A.S.,
Lanza A.F., Naponiello L., Affer L., Anche R.M., Andreuzzi G.,
Basilicata M., Brinjikji M., Brogi M., Cabona L., Carolo E., Colombo S.,
Damasso M., D'Arpa M., Di Filippo S., Harutyunyan A., Hom J., Mancini L.,
Mantovan G., Nardiello D., Santhakumari K.R., Zingales T.
<Astron. Astrophys. 707, A67 (2026)>
=2026A&A...707A..67P 2026A&A...707A..67P (SIMBAD/NED BibCode)
ADC_Keywords: Stars, double and multiple ; Stars, late-type ; Stars, M-type ;
Spectroscopy ; Exoplanets ; Radial velocities ; Optical
Keywords: methods: data analysis - techniques: radial velocities - astrometry -
planets and satellites: individual: HD 128717 b - brown dwarfs -
planetary systems
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 radial velocity
(RV) monitoring of HD128717, 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 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+0.06-0.05yr, M=19.8±0.5MJ,
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.
Description:
File datatable.dat provides the observational data collected with the
HARPS-N spectrograph for HD128717, and used in the present study.
Objects:
-------------------------==========≡=----------------
RA (2000) DE Designation(s)
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14 36 21.32 +57 33 38.4 HD 128717 = TIC 298163592
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
appena1.dat 98 106 Observational HARPS-N data
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Byte-by-byte Description of file: appena1.dat
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Bytes Format Units Label Explanations
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1- 12 F12.6 d BJD Observation date (BJD-2400000)
14- 24 F11.6 m/s RV Radial Velocity
26- 33 F8.6 m/s e_RV Radial Velocity error
35- 43 F9.6 --- RHK logR'HK
45- 52 F8.6 --- e_RHK logR'HK error
54- 62 F9.6 m/s FWHM ? Full Width Half Maximum of the CCF
64- 71 F8.6 m/s e_FWHM ? Full Width Half Maximum of the CCF error
73- 81 F9.6 m/s BIS ? Bisector span
83- 90 F8.6 m/s e_BIS ? Bisector span error
92- 98 A7 --- outliers Outliers identified in the analysis
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Acknowledgements:
Matteo Pinamonti, matteo.pinamonti(at)inaf.it
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(End) Matteo Pinamonti [INAF-OATO, Italy], Patricia Vannier [CDS] 06-Dec-2025