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1. The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. V. Do Self-consistent Atmospheric Models Represent JWST Spectra? A Showcase with VHS 1256–1257 b

   https://doi.org/10.3847/2041-8213/ad3e7c  

   Petrus, Simon; Whiteford, Niall; Patapis, Polychronis; Biller, Beth A; Skemer, Andrew; Hinkley, Sasha; Suárez, Genaro; Palma-Bifani, Paulina; Morley, Caroline V; Tremblin, Pascal; et al (April 2024, The Astrophysical Journal Letters)

   Abstract The unprecedented medium-resolution (R_λ∼ 1500–3500) near- and mid-infrared (1–18μm) spectrum provided by JWST for the young (140 ± 20 Myr) low-mass (12–20M_Jup) L–T transition (L7) companion VHS 1256 b gives access to a catalog of molecular absorptions. In this study, we present a comprehensive analysis of this data set utilizing a forward-modeling approach applying our Bayesian framework,ForMoSA. We explore five distinct atmospheric models to assess their performance in estimating key atmospheric parameters:T_eff, log(g), [M/H], C/O,γ,f_sed, andR. Our findings reveal that each parameter’s estimate is significantly influenced by factors such as the wavelength range considered and the model chosen for the fit. This is attributed to systematic errors in the models and their challenges in accurately replicating the complex atmospheric structure of VHS 1256 b, notably the complexity of its clouds and dust distribution. To propagate the impact of these systematic uncertainties on our atmospheric property estimates, we introduce innovative fitting methodologies based on independent fits performed on different spectral windows. We finally derived aT_effconsistent with the spectral type of the target, considering its young age, which is confirmed by our estimate of log(g). Despite the exceptional data quality, attaining robust estimates for chemical abundances [M/H] and C/O, often employed as indicators of formation history, remains challenging. Nevertheless, the pioneering case of JWST’s data for VHS 1256 b has paved the way for future acquisitions of substellar spectra that will be systematically analyzed to directly compare the properties of these objects and correct the systematics in the models. 

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2. Roaring Storms in the Planetary-mass Companion VHS 1256-1257 b: Hubble Space Telescope Multiepoch Monitoring Reveals Vigorous Evolution in an Ultracool Atmosphere

   https://doi.org/10.3847/1538-3881/ac9905  

   Zhou, Yifan; Bowler, Brendan P.; Apai, Dániel; Kataria, Tiffany; Morley, Caroline V.; Bryan, Marta L.; Skemer, Andrew J.; Benneke, Björn (November 2022, The Astronomical Journal)

   Abstract The photometric and spectral variability of brown dwarfs probes heterogeneous temperature and cloud distributions and traces the atmospheric circulation patterns. We present a new 42 hr Hubble Space Telescope (HST) Wide Field Camera 3 G141 spectral time series of VHS 1256-1257 b, a late L-type planetary-mass companion that has been shown to have one of the highest variability amplitudes among substellar objects. The light curve is rapidly evolving and best fit by a combination of three sine waves with different periods and a linear trend. The amplitudes of the sine waves and the linear slope vary with the wavelength, and the corresponding spectral variability patterns match the predictions by models invoking either heterogeneous clouds or thermal profile anomalies. Combining these observations with previous HST monitoring data, we find that the peak-to-valley flux difference is 33% ± 2% with an even higher amplitude reaching 38% in the J band, the highest amplitude ever observed in a substellar object. The observed light curve can be explained by maps that are composed of zonal waves, spots, or a mixture of the two. Distinguishing the origin of rapid light curve evolution requires additional long-term monitoring. Our findings underscore the essential role of atmospheric dynamics in shaping brown-dwarf atmospheres and highlight VHS 1256-1257 b as one of the most favorable targets for studying the atmospheres, clouds, and atmospheric circulation of planets and brown dwarfs. 

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3. Dynamical Masses of the Primary Be Star and Secondary sdB Star in the Single-lined Binary κ Dra (B6 IIIe)

   https://doi.org/10.3847/1538-4357/ac98b8  

   Klement, Robert; Baade, Dietrich; Rivinius, Thomas; Gies, Douglas R.; Wang, Luqian; Labadie-Bartz, Jonathan; dos Santos, Pedro Ticiani; Monnier, John D.; Carciofi, Alex C.; Mérand, Antoine; et al (November 2022, The Astrophysical Journal)

   Abstract Because many classical Be stars may owe their nature to mass and angular-momentum transfer in a close binary, the present masses, temperatures, and radii of their components are of high interest for comparison to stellar evolution models. ObjectκDra is a 61.5 day single-lined binary with a B6 IIIe primary. With the CHARA Array instruments MIRC/MIRC-X and MYSTIC, we detected the secondary at (approximately photospheric) flux ratios of 1.49% ± 0.10% and 1.63% ± 0.09% in theHandKband, respectively. From a large and diverse optical spectroscopic database, only the radial velocity curve of the Be star could be extracted. However, employing the parallaxes from Hipparcos and Gaia, which agree within their nominal 1σerrors, we could derive the total mass and found component masses of 3.65 ± 0.48 and 0.426 ± 0.043M_⊙for the Be star and the companion, respectively. Previous cross-correlation of the observed FUV spectrum with O-type subdwarf (sdO) spectral model templates had not detected a companion belonging to the hot sdO population known from ∼20 earlier-type Be stars. Guided by our full 3D orbital solution, we found a strong cross-correlation signal for a stripped subdwarf B-type companion (FUV flux ratio of 2.3% ± 0.5%), enabling the first firm characterization of such a star and makingκDra the first mid- to late-type Be star with a directly observed subdwarf companion. 

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4. Discovery of the Remarkably Red L/T Transition Object VHS J183135.58-551355.9

   https://doi.org/10.3847/1538-3881/ad4b16  

   Bickle, Thomas P; Schneider, Adam C; Gagné, Jonathan; Faherty, Jacqueline K; Rothermich, Austin; Vos, Johanna M; Suárez, Genaro; Kirkpatrick, J Davy; Meisner, Aaron M; Kuchner, Marc J; et al (July 2024, The Astronomical Journal)

   Abstract We present the discovery of VHS J183135.58−551355.9 (hereafter VHS J1831−5513), an L/T transition dwarf identified as a result of its unusually red near-infrared colors (J−K_S= 3.633 ± 0.277 mag;J−W2 = 6.249 ± 0.245 mag) from the VISTA Hemisphere Survey and CatWISE2020 surveys. We obtain low-resolution near-infrared spectroscopy of VHS J1831−5513 using the Magellan Folded port InfraRed Echellette spectrograph to confirm its extremely red nature and assess features sensitive to surface gravity (i.e., youth). Its near-infrared spectrum shows multiple CH₄absorption features, indicating an exceptionally low effective temperature for its spectral type. Based on proper-motion measurements from CatWISE2020 and a photometric distance derived from itsK_s-band magnitude, we find that VHS J1831−5513 is a likely (∼85% probability) kinematic member of theβPictoris moving group. Future radial velocity and trigonometric parallax measurements will clarify such membership. Follow-up mid-infrared or higher-resolution near-infrared spectroscopy of this object will allow for further investigation as to the cause(s) of its redness, such as youth, clouds, and viewing geometry. 

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5. Near-infrared Accretion Signatures from the Circumbinary Planetary-mass Companion Delorme 1 (AB)b*

   https://doi.org/10.3847/2041-8213/ac85ef  

   Betti, S. K.; Follette, K. B.; Ward-Duong, K.; Aoyama, Y.; Marleau, G. -D.; Bary, J.; Robinson, C.; Janson, M.; Balmer, W.; Chauvin, G.; et al (August 2022, The Astrophysical Journal Letters)

   Abstract Accretion signatures from bound brown dwarf and protoplanetary companions provide evidence for ongoing planet formation, and accreting substellar objects have enabled new avenues to study the astrophysical mechanisms controlling the formation and accretion processes. Delorme 1 (AB)b, a ∼30–45 Myr circumbinary planetary-mass companion, was recently discovered to exhibit strong Hαemission. This suggests ongoing accretion from a circumplanetary disk, somewhat surprising given canonical gas disk dispersal timescales of 5–10 Myr. Here, we present the first NIR detection of accretion from the companion in Paβ, Paγ, and Brγemission lines from SOAR/TripleSpec 4.1, confirming and further informing its accreting nature. The companion shows strong line emission, withL_line≈ 1–6 × 10⁻⁸L_⊙across lines and epochs, while the binary host system shows no NIR hydrogen line emission (L_line< 0.32–11 × 10⁻⁷L_⊙). Observed NIR hydrogen line ratios are more consistent with a planetary accretion shock than with local line excitation models commonly used to interpret stellar magnetospheric accretion. Using planetary accretion shock models, we derive mass accretion rate estimates of ${\dot{M}}_{\mathrm{pla}}\sim 3$–4 × 10⁻⁸M_Jyr⁻¹, somewhat higher than expected under the standard star formation paradigm. Delorme 1 (AB)b’s high accretion rate is perhaps more consistent with formation via disk fragmentation. Delorme 1 (AB)b is the first protoplanet candidate with clear (signal-to-noise ratio ∼5) NIR hydrogen line emission. 

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