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1. Protosolar D-to-H Abundance and One Part per Billion PH ₃ in the Coldest Brown Dwarf

   https://doi.org/10.3847/2041-8213/ad9744  

   Rowland, Melanie_J; Morley, Caroline_V; Miles, Brittany_E; Suarez, Genaro; Faherty, Jacqueline_K; Skemer, Andrew_J; Beiler, Samuel_A; Line, Michael_R; Bjoraker, Gordon_L; Fortney, Jonathan_J; et al (December 2024, The Astrophysical Journal Letters)

   Abstract The coldest Y spectral type brown dwarfs are similar in mass and temperature to cool and warm (∼200–400 K) giant exoplanets. We can therefore use their atmospheres as proxies for planetary atmospheres, testing our understanding of physics and chemistry for these complex, cool worlds. At these cold temperatures, their atmospheres are cold enough for water clouds to form, and chemical timescales increase, increasing the likelihood of disequilibrium chemistry compared to warmer classes of planets. JWST observations are revolutionizing the characterization of these worlds with high signal-to-noise, moderate-resolution near- and mid-infrared spectra. The spectra have been used to measure the abundances of prominent species, like water, methane, and ammonia; species that trace chemical reactions, like carbon monoxide; and even isotopologues of carbon monoxide and ammonia. Here, we present atmospheric retrieval results using both published fixed-slit (Guaranteed Time Observation program 1230) and new averaged time series observations (GO program 2327) of the coldest known Y dwarf, WISE 0855–0714 (using NIRSpec G395M spectra), which has an effective temperature of ∼264 K. We present a detection of deuterium in an atmosphere outside of the solar system via a relative measurement of deuterated methane (CH₃D) and standard methane. From this, we infer the D/H ratio of a substellar object outside the solar system for the first time. We also present a well-constrained part-per-billion abundance of phosphine (PH₃). We discuss our interpretation of these results and the implications for brown dwarf and giant exoplanet formation and evolution. 

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2. Importance of gas heating in capacitively coupled radiofrequency plasma-assisted synthesis of carbon nanomaterials

   https://doi.org/10.1088/1361-6463/ad6d78  

   Nikhar, Tanvi; Basu, Sankhadeep; Abe, Shota; Yatom, Shurik; Raitses, Yevgeny; Anthony, Rebecca; Baryshev, Sergey V (August 2024, Journal of Physics D: Applied Physics)

   Abstract In pursuit of diamond nanoparticles, a capacitively-coupled radio frequency flow-through plasma reactor was operated with methane-argon gas mixtures. Signatures of the final product obtained microscopically and spectroscopically indicated that the product was an amorphous form of graphite. This result was consistent irrespective of combinations of the macroscopic reactor settings. To explain the observed synthesis output, measurements of C₂and gas properties were carried out by laser-induced fluorescence and optical emission spectroscopy. Strikingly, the results indicated a strong gas temperature gradient of 100 K per mm from the center of the reactor to the wall. Based on additional plasma imaging, a model of hot constricted region (filamentation region) was then formulated. It illustrated that, while the hot constricted region was present, the bulk of the gas was not hot enough to facilitate diamondsp³formation: characterized by much lower reaction rates, when compared tosp²,sp³formation kinetics are expected to become exponentially slow. This result was further confirmed by experiments under identical conditions but with a H₂/CH₄mixture, where no output material was detected: if graphiticsp²formation was expected as the main output material from the methane feedstock, atomic hydrogen would then be expected to etch it awayin situ, such that the net production of thatsp²-hybridized solid material is nearly a zero. Finally, the crucial importance of gas heating was corroborated by replacing RF with microwave source whereby facilesp³production was attained with H₂/CH₄gas mixture. 

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3. CWISE J105512.11+544328.3: A Nearby Y Dwarf Spectroscopically Confirmed with Keck/NIRES

   https://doi.org/10.3847/1538-4357/ad0043  

   Robbins, Grady; Meisner, Aaron M.; Schneider, Adam C.; Burgasser, Adam J.; Kirkpatrick, J. Davy; Gagné, Jonathan; Hsu, Chih-Chun; Moranta, Leslie; Casewell, Sarah; Marocco, Federico; et al (November 2023, The Astrophysical Journal)

   Abstract Y dwarfs, the coolest known spectral class of brown dwarfs, overlap in mass and temperature with giant exoplanets, providing unique laboratories for studying low-temperature atmospheres. However, only a fraction of Y dwarf candidates have been spectroscopically confirmed. We present Keck/NIRES near-infrared spectroscopy of the nearby (d≈ 6–8 pc) brown dwarf CWISE J105512.11+544328.3. Although its near-infrared spectrum aligns best with the Y0 standard in theJband, no standard matches well across the fullYJHKwavelength range. The CWISE J105512.11+544328.3 NH₃-H= 0.427 ± 0.0012 and CH₄-J= 0.0385 ± 0.0007 absorption indices and absolute Spitzer [4.5] magnitude of 15.18 ± 0.22 are also indicative of an early-Y dwarf rather than a late-T dwarf. CWISE J105512.11+544328.3 additionally exhibits the bluest Spitzer [3.6]−[4.5] color among all spectroscopically confirmed Y dwarfs. Despite this anomalously blue Spitzer color given its low luminosity, CWISE J105512.11+544328.3 does not show other clear kinematic or spectral indications of low metallicity. Atmospheric model comparisons yield a log(g) ≤ 4.5 andT_eff≈ 500 ± 150 K for this source. We classify CWISE J105512.11+544328.3 as a Y0 (pec) dwarf, adding to the remarkable diversity of the Y-type population. JWST spectroscopy would be crucial to understanding the origin of this Y dwarf’s unusual preference for low-gravity models and blue 3–5μm color. 

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4. CONSTRAINTS ON AURORAL RADIO EMISSION FROM Y DWARFS

   Melodie M. Kao, Gregg Hallinan (April 2018, Astrophysical journal)

   As an initial pilot study of magnetism in Y dwarfs, we have observed the three known IR variable Y dwarfs WISE J085510.83-071442.5, WISE J140518.40+553421.4, and WISEP J173835.53+273258.9 with the Karl G. Jansky Very Large Array (VLA) from 4-8 GHz to investigate the presence of quiescent radio emission as a proxy for highly circularly polarized radio emission associated with large-scale auroral currents. Measurements of magnetic fields on Y dwarfs, currently only possible by observing auroral radio emission, are essential for constraining fully convective magnetic dynamo models. We do not detect any pulsed or quiescent radio emission, down to rms noise levels of 7.2 uJy for WISE J085510.83-071442.5, 2.2 uJy for WISE J140518.40+553421.4, and 3.2 uJy for WISEP J173835.53+273258.9. The fractional detection rate of radio emission from T dwarfs is <10% and suggests that a much larger sample of deep observations of Y dwarfs is needed to rule out radio emission in the Y dwarf population. The significance of a single detection provides strong motivation for such a search. 

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5. A broadband thermal emission spectrum of the ultra-hot Jupiter WASP-18b

   https://doi.org/10.1038/s41586-023-06230-1  

   Coulombe, Louis-Philippe; Benneke, Björn; Challener, Ryan; Piette, Anjali_A A; Wiser, Lindsey S; Mansfield, Megan; MacDonald, Ryan J; Beltz, Hayley; Feinstein, Adina D; Radica, Michael; et al (August 2023, Nature)

   Abstract Close-in giant exoplanets with temperatures greater than 2,000 K (‘ultra-hot Jupiters’) have been the subject of extensive efforts to determine their atmospheric properties using thermal emission measurements from the Hubble Space Telescope (HST) and Spitzer Space Telescope^1–3. However, previous studies have yielded inconsistent results because the small sizes of the spectral features and the limited information content of the data resulted in high sensitivity to the varying assumptions made in the treatment of instrument systematics and the atmospheric retrieval analysis^3–12. Here we present a dayside thermal emission spectrum of the ultra-hot Jupiter WASP-18b obtained with the NIRISS¹³instrument on the JWST. The data span 0.85 to 2.85 μm in wavelength at an average resolving power of 400 and exhibit minimal systematics. The spectrum shows three water emission features (at >6σconfidence) and evidence for optical opacity, possibly attributable to H⁻, TiO and VO (combined significance of 3.8σ). Models that fit the data require a thermal inversion, molecular dissociation as predicted by chemical equilibrium, a solar heavy-element abundance (‘metallicity’,$${\rm{M/H}}=1.0{3}_{-0.51}^{+1.11}$$ $\mathrm{M/H}=1.03_{-0.51}^{+1.11}$times solar) and a carbon-to-oxygen (C/O) ratio less than unity. The data also yield a dayside brightness temperature map, which shows a peak in temperature near the substellar point that decreases steeply and symmetrically with longitude towards the terminators. 

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