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1. 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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2. Long-term 4.6 μm Variability in Brown Dwarfs and a New Technique for Identifying Brown Dwarf Binary Candidates

   https://doi.org/10.3847/1538-3881/acccf7  

   Brooks, Hunter; Kirkpatrick, J. Davy; Meisner, Aaron M.; Gelino, Christopher R.; Bardalez Gagliuffi, Daniella C.; Marocco, Federico; Schneider, Adam C.; Faherty, Jacqueline K.; Casewell, S. L.; Raghu, Yadukrishna; et al (May 2023, The Astronomical Journal)

   Abstract Using a sample of 361 nearby brown dwarfs, we have searched for 4.6μm variability, indicative of large-scale rotational modulations or large-scale, long-term changes on timescales of over 10 yr. Our findings show no statistically significant variability in Spitzer’s Infrared Array Camera (IRAC) channel 2 (ch2) or Wide-field Infrared Survey Explorer W2 photometry. For Spitzer the ch2 1σlimits are ∼8 mmag for objects at 11.5 mag and ∼22 mmag for objects at 16 mag. This corresponds to no variability above 4.5% at 11.5 mag and 12.5% at 16 mag. We conclude that highly variable brown dwarfs, at least two previously published examples of which have been shown to have 4.6μm variability above 80 mmag, are very rare. While analyzing the data, we also developed a new technique for identifying brown dwarf binary candidates in Spitzer data. We find that known binaries have IRAC ch2 point response function (PRF) flux measurements that are consistently dimmer than aperture flux measurements. We have identified 59 objects that exhibit such PRF versus aperture flux differences and are thus excellent binary brown dwarf candidates. 

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3. Search for globular clusters associated with the Milky Way dwarf galaxies using Gaia DR2

   https://doi.org/10.1093/mnras/staa3297  

   Huang, Kuan-Wei; Koposov, Sergey E (November 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We report the result of searching for globular clusters (GCs) around 55 Milky Way (MW) satellite dwarf galaxies within the distance of 450 kpc from the Galactic Centre except for the Large and Small Magellanic Clouds and the Sagittarius dwarf. For each dwarf, we analyse the stellar distribution of sources in Gaia DR2, selected by magnitude, proper motion, and source morphology. Using the kernel density estimation of stellar number counts, we identify 11 possible GC candidates. Cross-matched with existing imaging data, all 11 objects are known either GCs or galaxies and only Fornax GC 1–6 among them are associated with the targeted dwarf galaxy. Using simulated GCs, we calculate the GC detection limit $$M_{\rm V}^{\rm lim}$$ that spans the range from $$M_{\rm V}^{\rm lim}\sim -7$$ for distant dwarfs to $$M_{\rm V}^{\rm lim}\sim 0$$ for nearby systems. Assuming a Gaussian GC luminosity function, we compute that the completeness of the GC search is above 90 per cent for most dwarf galaxies. We construct the 90 per cent credible intervals/upper limits on the GC specific frequency SN of the MW dwarf galaxies: 12 < SN < 47 for Fornax, SN < 20 for the dwarfs with −12 < MV < −10, SN < 30 for the dwarfs with −10 < MV < −7, and SN < 90 for the dwarfs with MV > −7. Based on SN, we derive the probability of galaxies hosting GCs given their luminosity, finding that the probability of galaxies fainter than MV = −9 to host GCs is lower than 0.1. 

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4. Discovery of 118 New Ultracool Dwarf Candidates Using Machine-learning Techniques

   https://doi.org/10.3847/1538-3881/ad77d2  

   Brooks, Hunter; Caselden, Dan; Kirkpatrick, J Davy; Raghu, Yadukrishna; Elachi, Charles A; Grigorian, Jake; Trek, Asa; Washburn, Andrew; Higashimura_東, Hiro 村滉; Meisner, Aaron M; et al (October 2024, The Astronomical Journal)

   Abstract We present the discovery of 118 new ultracool dwarf candidates, discovered using a new machine-learning tool, namedSMDET, applied to time-series images from the Wide-field Infrared Survey Explorer. We gathered photometric and astrometric data to estimate each candidate’s spectral type, distance, and tangential velocity. This sample has a photometrically estimated spectral class distribution of 28 M dwarfs, 64 L dwarfs, and 18 T dwarfs. We also identify a T-subdwarf candidate, two extreme T-subdwarf candidates, and two candidate young ultracool dwarfs. Five objects did not have enough photometric data for any estimations to be made. To validate our estimated spectral types, spectra were collected for two objects, yielding confirmed spectral types of T5 (estimated T5) and T3 (estimated T4). Demonstrating the effectiveness of machine-learning tools as a new large-scale discovery technique. 

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5. High Proper Motion Discoveries from the UKIRT Hemisphere Survey

   https://doi.org/10.3847/1538-3881/ae0ad5  

   Zhang, Wings; Schneider, Adam C; Bickle, Thomas P; Burgasser, Adam J; Softich, Emma; Marocco, Federico; Bardalez_Gagliuffi, Daniella; Faherty, Jacqueline K; Meisner, Aaron M; Kirkpatrick, J Davy; et al (October 2025, The Astronomical Journal)

   Abstract We used the third data release of the UKIRT Hemisphere Survey to locate previously unrecognized high proper motion objects. We identify a total of 127 new discoveries with total proper motions ≳300 mas yr⁻¹. A significant fraction of these sources with counterparts in the Gaia DR3 catalog are found to be distant (>100 pc) low-mass stars, where their large tangential velocities and placement on color–magnitude diagrams suggest that they are likely low-metallicity M-type subdwarfs. Optical spectroscopy is used to confirm the low mass and low metallicity for two such sources. Using available optical and infrared photometry, we estimate the spectral type for all non-Gaia sources and find 10 likely late-M dwarfs, 15 objects with colors most consistent with L-type dwarfs, and nine possible T-type dwarfs. Follow-up spectroscopy is needed to confirm spectral types and further characterize these new discoveries. 

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