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1. A Clear View of a Cloudy Brown Dwarf Companion from High-resolution Spectroscopy

   https://doi.org/10.3847/1538-4357/ac8673  

   Xuan, Jerry W.; Wang, Jason; Ruffio, Jean-Baptiste; Knutson, Heather; Mawet, Dimitri; Mollière, Paul; Kolecki, Jared; Vigan, Arthur; Mukherjee, Sagnick; Wallack, Nicole; et al (September 2022, The Astrophysical Journal)

   Abstract Direct imaging studies have mainly used low-resolution spectroscopy ( R ∼ 20–100) to study the atmospheres of giant exoplanets and brown dwarf companions, but the presence of clouds has often led to degeneracies in the retrieved atmospheric abundances (e.g., carbon-to-oxygen ratio, metallicity). This precludes clear insights into the formation mechanisms of these companions. The Keck Planet Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to transport light into NIRSPEC ( R ∼ 35,000 in the K band), and aims to address these challenges with high-resolution spectroscopy. Using an atmospheric retrieval framework based on petitRADTRANS , we analyze the KPIC high-resolution spectrum (2.29–2.49 μ m) and the archival low-resolution spectrum (1–2.2 μ m) of the benchmark brown dwarf HD 4747 B ( m = 67.2 ± 1.8 M Jup , a = 10.0 ± 0.2 au, T eff ≈ 1400 K). We find that our measured C/O and metallicity for the companion from the KPIC high-resolution spectrum agree with those of its host star within 1 σ –2 σ . The retrieved parameters from the K -band high-resolution spectrum are also independent of our choice of cloud model. In contrast, the retrieved parameters from the low-resolution spectrum are highly sensitive to our chosen cloud model. Finally, we detect CO, H 2 O, and CH 4 (volume-mixing ratio of log(CH 4 ) = −4.82 ± 0.23) in this L/T transition companion with the KPIC data. The relative molecular abundances allow us to constrain the degree of chemical disequilibrium in the atmosphere of HD 4747 B, and infer a vertical diffusion coefficient that is at the upper limit predicted from mixing length theory. 

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2. Direct imaging and dynamical mass of a benchmark T-type brown dwarf companion to HD 167665

   https://doi.org/10.1051/0004-6361/202451184  

   Maire, A-L; Leclerc, A; Balmer, W O; Desidera, S; Lacour, S; D’Orazi, V; Samland, M; Langlois, M; Matthews, E; Babusiaux, C; et al (November 2024, Astronomy & Astrophysics)

   Context. A low-mass companion potentially in the brown dwarf mass regime was discovered on a ~12 yr orbit (~5.5 au) around HD 167665 using radial velocity (RV) monitoring. Joint RV–astrometry analyses confirmed that HD 167665B is a brown dwarf with precisions on the measured mass of ~4–9%. Brown dwarf companions with measured mass and luminosity are valuable for testing formation and evolutionary models. However, its atmospheric properties and luminosity are still unconstrained, preventing detailed tests of evolutionary models. Aims. We further characterize the HD 167665 system by measuring the luminosity and refining the mass of its companion and reassessing the stellar age. Methods. We present new high-contrast imaging data of the star and of its close-in environment from SPHERE and GRAVITY, which we combined with RV data from CORALIE and HIRES and astrometry from HIPPARCOSandGaia. Results. The analysis of the host star properties indicates an age of 6.20 ± 1.13 Gyr. GRAVITY reveals a point source near the position predicted from a joint fit of RV data and HIPPARCOS–Gaiaproper motion anomalies. Subsequent SPHERE imaging confirms the detection and reveals a faint point source of contrast of ∆H2= 10.95 ± 0.33 mag at a projected angular separation of ~180 mas. A joint fit of the high-contrast imaging, RV, and HIPPARCOSintermediate astrometric data together with theGaiaastrometric parameters constrains the mass of HD 167665B to ~1.2%, 60.3 ± 0.7M_J. The SPHERE colors and spectrum point to an early or mid-T brown dwarf of spectral type T4₋₂⁺¹. Fitting the SPHERE spectrophotometry and GRAVITY spectrum with synthetic spectra suggests an effective temperature of ~1000–1150 K, a surface gravity of ~5.0–5.4 dex, and a bolometric luminosity log(L/L_⊙)=−4.892_−0.028^+0.024dex. The mass, luminosity, and age of the companion can only be reproduced within 3σby the hybrid cloudy evolutionary models of Saumon & Marley (2008, ApJ, 689, 1327), whereas cloudless evolutionary models underpredict its luminosity. 

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3. Let the Great World Spin: Revealing the Stormy, Turbulent Nature of Young Giant Exoplanet Analogs with the Spitzer Space Telescope

   https://doi.org/10.3847/1538-4357/ac4502  

   Vos, Johanna M.; Faherty, Jacqueline K.; Gagné, Jonathan; Marley, Mark; Metchev, Stanimir; Gizis, John; Rice, Emily L.; Cruz, Kelle (January 2022, The Astrophysical Journal)

   Abstract We present a survey for photometric variability in young, low-mass brown dwarfs with the Spitzer Space Telescope. The 23 objects in our sample show robust signatures of youth and share properties with directly imaged exoplanets. We present three new young objects: 2MASS J03492367+0635078, 2MASS J09512690−8023553, and 2MASS J07180871−6415310. We detect variability in 13 young objects, and find that young brown dwarfs are highly likely to display variability across the L2–T4 spectral type range. In contrast, the field dwarf variability occurrence rate drops for spectral types >L9. We examine the variability amplitudes of young objects and find an enhancement in maximum amplitudes compared to field dwarfs. We speculate that the observed range of amplitudes within a spectral type may be influenced by secondary effects such as viewing inclination and/or rotation period. We combine our new rotation periods with the literature to investigate the effects of mass on angular momentum evolution. While high-mass brown dwarfs (>30 M Jup ) spin up over time, the same trend is not apparent for lower-mass objects (<30 M Jup ), likely due to the small number of measured periods for old, low-mass objects. The rotation periods of companion brown dwarfs and planetary-mass objects are consistent with those of isolated objects with similar ages and masses, suggesting similar angular momentum histories. Within the AB Doradus group, we find a high-variability occurrence rate and evidence for common angular momentum evolution. The results are encouraging for future variability searches in directly imaged exoplanets with facilities such as the James Webb Space Telescope and 30 m telescopes. 

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4. VLTI/GRAVITY Observations and Characterization of the Brown Dwarf Companion HD 72946 B

   https://doi.org/10.3847/1538-4357/acf761  

   Balmer, William O; Pueyo, Laurent; Stolker, Tomas; Reggiani, Henrique; Maire, A-L; Lacour, S; Mollière, P; Nowak, M; Sing, D; Pourré, N; et al (October 2023, The Astrophysical Journal)

   Abstract Tension remains between the observed and modeled properties of substellar objects, but objects in binary orbits, with known dynamical masses, can provide a way forward. HD 72946 B is a recently imaged brown dwarf companion to a nearby, solar-type star. We achieve ∼100μas relative astrometry of HD 72946 B in theKband using VLTI/GRAVITY, unprecedented for a benchmark brown dwarf. We fit an ensemble of measurements of the orbit usingorbitize!and derive a strong dynamical mass constraintM_B= 69.5 ± 0.5M_Jupassuming a strong prior on the host star massM_A= 0.97 ± 0.01M_⊙from an updated stellar analysis. We fit the spectrum of the companion to a grid of self-consistentBT-Settl-CIFISTmodel atmospheres, and perform atmospheric retrievals usingpetitRADTRANS. A dynamical mass prior only marginally influences the sampled distribution of effective temperature, but has a large influence on the surface gravity and radius, as expected. The dynamical mass alone does not strongly influence retrieved pressure–temperature or cloud parameters within our current retrieval setup. Independently of the cloud prescription and prior assumptions, we find agreement within ±2σbetween the C/O of the host (0.52 ± 0.05) and brown dwarf (0.43–0.63), as expected from a molecular cloud collapse formation scenario, but our retrieved metallicities are implausibly high (0.6–0.8) in light of the excellent agreement of the data with the solar-abundance model grid. Future work on our retrieval framework will seek to resolve this tension. Additional study of low surface gravity objects is necessary to assess the influence of a dynamical mass prior on atmospheric analysis. 

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5. Stellar Obliquities of Young Systems, Atmospheres Undergoing Contraction and Escape (SOYSAUCE): A Likely Aligned Orbit for the 3 Myr Planet TIDYE-1 b

   https://doi.org/10.3847/2041-8213/ae2004  

   Barber, Madyson G; Mann, Andrew W; Johnson, Marshall C; Mori, Mayuko; Livingston, John; Krolikowski, Daniel M; Narita, Norio; Fukui, Akihiko; Hirano, Teruyuki; Vanderburg, Andrew; et al (November 2025, The Astrophysical Journal Letters)

   Abstract Despite the wide range of planet–star (mis)alignments in the mature population of transiting exoplanets, the small number of known young transiting planets are nearly all aligned with the rotation axes of their host stars, as determined by the sky-projected obliquity angle. The small number of young systems with measured obliquities limits statistical conclusions. Here we determine the sky-projected obliquity (λ) of the 3 Myr transiting planet with a misaligned outer protoplanetary disk, TIDYE-1 b (IRAS 04125+2902b), using the Rossiter–McLaughlin (RM) effect. Our dataset lacks a pretransit baseline and ingress, complicating a blind RM fit. Instead, we use contemporaneous spectra and photometry from a mass-measurement campaign to model the stellar activity trend across the transit and provide an external prior on the velocity baseline. We determine $\mid \lambda \mid =11.8_{-5.0}^{+5.9}°$ . Combined with the published rotational velocity of the star, we find a true three-dimensional obliquity of $\psi =15.2_{-5.7}^{+7.3}°$ . Our result is consistent with an aligned orbit, suggesting the planet remains aligned to its star even though the outer disk is misaligned, though additional RM observations are needed to exclude the low-probability tail of misaligned (>30°) scenarios present in our posterior. 

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