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1. Direct discovery of the inner exoplanet in the HD 206893 system: Evidence for deuterium burning in a planetary-mass companion

   https://doi.org/10.1051/0004-6361/202244727  

   Hinkley, S.; Lacour, S.; Marleau, G.-D.; Lagrange, A.-M.; Wang, J. J.; Kammerer, J.; Cumming, A.; Nowak, M.; Rodet, L.; Stolker, T.; et al (March 2023, Astronomy & Astrophysics)

   Aims. HD 206893 is a nearby debris disk star that hosts a previously identified brown dwarf companion with an orbital separation of ∼10 au. Long-term precise radial velocity (RV) monitoring, as well as anomalies in the system proper motion, has suggested the presence of an additional, inner companion in the system. Methods. Using information from ongoing precision RV measurements with the HARPS spectrograph, as well as Gaia host star astrometry, we have undertaken a multi-epoch search for the purported additional planet using the VLTI/GRAVITY instrument. Results. We report a high-significance detection over three epochs of the companion HD 206893c, which shows clear evidence for Keplerian orbital motion. Our astrometry with ∼50−100 μarcsec precision afforded by GRAVITY allows us to derive a dynamical mass of 12.7$$ ^{+1.2}_{-1.0} $$ M Jup and an orbital separation of 3.53$$ ^{+0.08}_{-0.06} $$ au for HD 206893c. Our fits to the orbits of both companions in the system use both Gaia astrometry and RVs to also provide a precise dynamical estimate of the previously uncertain mass of the B component, and therefore allow us to derive an age of 155 ± 15 Myr for the system. We find that theoretical atmospheric and evolutionary models that incorporate deuterium burning for HD 206893c, parameterized by cloudy atmosphere models as well as a “hybrid sequence” (encompassing a transition from cloudy to cloud-free), provide a good simultaneous fit to the luminosity of both HD 206893B and c. Thus, accounting for both deuterium burning and clouds is crucial to understanding the luminosity evolution of HD 206893c. Conclusions. In addition to using long-term RV information, this effort is an early example of a direct imaging discovery of a bona fide exoplanet that was guided in part by Gaia astrometry. Utilizing Gaia astrometry is expected to be one of the primary techniques going forward for identifying and characterizing additional directly imaged planets. In addition, HD 206893c is an example of an object narrowly straddling the deuterium-burning limit but unambiguously undergoing deuterium burning. Additional discoveries like this may therefore help clarify the discrimination between a brown dwarf and an extrasolar planet. Lastly, this discovery is another example of the power of optical interferometry to directly detect and characterize extrasolar planets where they form, at ice-line orbital separations of 2−4 au. 

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2. GRAVITY K -band spectroscopy of HD 206893 B: Brown dwarf or exoplanet

   https://doi.org/10.1051/0004-6361/202140749  

   Kammerer, J; Lacour, S; Stolker, T; Mollière, P; Sing, D K; Nasedkin, E; Kervella, P; Wang, J J; Ward-Duong, K; Nowak, M; et al (August 2021, Astronomy & Astrophysics)

   Context.Near-infrared interferometry has become a powerful tool for studying the orbital and atmospheric parameters of substellar companions. Aims.We aim to reveal the nature of the reddest known substellar companion HD 206893 B by studying its near-infrared colors and spectral morphology and by investigating its orbital motion. Methods.We fit atmospheric models for giant planets and brown dwarfs and perform spectral retrievals withpetitRADTRANSandATMOon the observed GRAVITY, SPHERE, and GPI spectra of HD 206893 B. To recover its unusual spectral features, first and foremost its extremely red near-infrared color, we include additional extinction by high-altitude dust clouds made of enstatite grains in the atmospheric model fits. However, forsterite, corundum, and iron grains predict similar extinction curves for the grain sizes considered here. We also infer the orbital parameters of HD 206893 B by combining the ~100 μas precision astrometry from GRAVITY with data from the literature and constrain the mass and position of HD 206893 C based on theGaiaproper motion anomaly of the system. Results.The extremely red color and the very shallow 1.4 μm water absorption feature of HD 206893 B can be fit well with the adapted atmospheric models and spectral retrievals. By comparison with AMES-Cond evolutionary tracks, we find that only some atmosphericmodels predict physically plausible objects. Altogether, our analysis suggests an age of ~ 3–300 Myr and a mass of ~ 5–30 M_Jupfor HD 206893 B, which is consistent with previous estimates but extends the parameter space to younger and lower-mass objects. The GRAVITY astrometry points to an eccentric orbit (e = 0.29_−0.11^+0.06) with a mutual inclination of <34.4 deg with respectto the debris disk of the system. Conclusions.While HD 206893 B could in principle be a planetary-mass companion, this possibility hinges on the unknown influence of the inner companion on the mass estimate of 10₋₄⁺⁵ M_Jupfrom radial velocity andGaiaas well as a relatively small but significant Argus moving group membership probability of ~ 61%. However, we find that if the mass of HD 206893 B is <30 M_Jup, then the inner companion HD 206893 C should have a mass between ~ 8–15 M_Jup. Finally, further spectroscopic or photometric observations at higher signal-to-noise and longer wavelengths are required to learn more about the composition and dust cloud properties of HD 206893 B. 

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3. The ExoGRAVITY survey: A K -band spectral library of giant exoplanet and brown dwarf companions

   https://doi.org/10.1051/0004-6361/202556860  

   Kammerer, J; Winterhalder, T O; Lacour, S; Stolker, T; Marleau, G-D; Balmer, W O; Moore, A F; Piscarreta, L; Toci, C; Mérand, A; et al (December 2025, Astronomy & Astrophysics)

   Context. Direct observations of exoplanet and brown dwarf companions with near-infrared interferometry, first enabled by the dualfield mode of VLTI/GRAVITY, provide unique measurements of the objects’ orbital motions and atmospheric compositions. Aims. Here we compile a homogeneous library of all exoplanet and brown dwarfK-band spectra observed by GRAVITY thus far. This ExoGRAVITY Spectral Library is made publicly available online. Methods. We re-reduced all the available GRAVITY dual-field high-contrast data in a uniform and highly automated way and, where companions were detected, extracted their ~2.0-2.4 μmK-band contrast spectra. We then derived stellar model atmospheres for all the employed flux references (either the host star or the swap calibrator), which we used to convert the companion contrast into companion flux spectra. Solely from the resulting GRAVITYK-band flux spectra, we extracted spectral types, spectral indices, and bulk physical properties for all the companions. Finally, and with the help of age constraints from the literature, we also derived isochronal masses for most of the companions using evolutionary models. Results. The resulting library containsR~ 500 GRAVITYK-band spectra of 39 substellar companions from late M to late T spectral types, including the entire L-T transition. Throughout this transition, a shift from CO-dominated late M- and L-type dwarfs to CH₄-dominated T-type dwarfs can be observed in theK-band. The GRAVITY spectra alone constrain the objects’ bolometric luminosity to typically within ±0.15 dex. The derived isochronal masses agree with dynamical masses from the literature where available, except for HD 4113 c for which we confirm its previously reported potential underluminosity. Conclusions. Medium-resolution spectroscopy of substellar companions with GRAVITY provides insight into the carbon chemistry and the cloudiness of these objects’ atmospheres. It also constrains these objects’ bolometric luminosities, which can yield measurements of their formation entropy if combined with dynamical masses, for instance from Gaia and GRAVITY astrometry. 

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4. Gaia-4b and 5b: Radial Velocity Confirmation of Gaia Astrometric Orbital Solutions Reveal a Massive Planet and a Brown Dwarf Orbiting Low-mass Stars

   https://doi.org/10.3847/1538-3881/ada9e1  

   Stefánsson, Gudmundur; Mahadevan, Suvrath; Winn, Joshua N; Marcussen, Marcus L; Kanodia, Shubham; Albrecht, Simon; Fitzmaurice, Evan; Mikulskytė, Onė; Cañas, Caleb I; Espinoza-Retamal, Juan I; et al (February 2025, The Astronomical Journal)

   Abstract Gaia astrometry of nearby stars is precise enough to detect the tiny displacements induced by substellar companions, but radial velocity (RV) data are needed for definitive confirmation. Here we present RV follow-up observations of 28 M and K stars with candidate astrometric substellar companions, which led to the confirmation of two systems, Gaia-4b and Gaia-5b, identification of five systems that are single lined but require additional data to confirm as substellar companions, and the refutation of 21 systems as stellar binaries. Gaia-4b is a massive planet (M = 11.8 ± 0.7M_J) in aP = 571.3 ± 1.4 day orbit with a projected semimajor axisa₀ = 0.312 ± 0.040 mas orbiting a 0.644 ± 0.02M_⊙star. Gaia-5b is a brown dwarf (M = 20.9 ± 0.5M_J) in aP = 358.62 ± 0.20 days eccentrice = 0.6423 ± 0.0026 orbit with a projected angular semimajor axis ofa₀ = 0.947 ± 0.038 mas around a 0.34 ± 0.03M_⊙star. Gaia-4b is one of the first exoplanets discovered via the astrometric technique, and is one of the most massive planets known to orbit a low-mass star. 

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5. High contrast at short separation with VLTI/GRAVITY: Bringing Gaia companions to light

   https://doi.org/10.1051/0004-6361/202449507  

   Pourré, N; Winterhalder, T O; Le_Bouquin, J-B; Lacour, S; Bidot, A; Nowak, M; Maire, A-L; Mouillet, D; Babusiaux, C; Woillez, J; et al (June 2024, Astronomy & Astrophysics)

   Context.Since 2019, GRAVITY has provided direct observations of giant planets and brown dwarfs at separations of down to 95 mas from the host star. Some of these observations have provided the first direct confirmation of companions previously detected by indirect techniques (astrometry and radial velocities). Aims.We want to improve the observing strategy and data reduction in order to lower the inner working angle of GRAVITY in dual-field on-axis mode. We also want to determine the current limitations of the instrument when observing faint companions with separations in the 30–150 mas range. Methods.To improve the inner working angle, we propose a fiber off-pointing strategy during the observations to maximize the ratio of companion-light-to-star-light coupling in the science fiber. We also tested a lower-order model for speckles to decouple the companion light from the star light. We then evaluated the detection limits of GRAVITY using planet injection and retrieval in representative archival data. We compare our results to theoretical expectations. Results.We validate our observing and data-reduction strategy with on-sky observations; first in the context of brown dwarf follow-up on the auxiliary telescopes with HD 984 B, and second with the first confirmation of a substellar candidate around the starGaiaDR3 2728129004119806464. With synthetic companion injection, we demonstrate that the instrument can detect companions down to a contrast of 8 × 10⁻⁴(ΔΚ= 7.7 mag) at a separation of 35 mas, and a contrast of 3 × 10⁻⁵(ΔΚ= 11 mag) at 100 mas from a bright primary (K< 6.5), for 30 min exposure time. Conclusions.With its inner working angle and astrometric precision, GRAVITY has a unique reach in direct observation parameter space. This study demonstrates the promising synergies between GRAVITY andGaiafor the confirmation and characterization of substellar companions. 

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