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1. Discovery of a Low-mass Comoving System Using NOIRLab Source Catalog DR2

   https://doi.org/10.3847/2515-5172/ac1f9c  

   Kiwy, Frank ; Faherty, Jacqueline ; Meisner, Aaron ; Schneider, Adam C. ; Kuchner, Marc ; Kirkpatrick, J. Davy ( August 2021 , Research Notes of the AAS)

   Abstract We present the discovery of a low-mass comoving system found by means of the NOIRLab Source Catalog DR2. The system consists of the high proper-motion star LEHPM 5005 and an ultracool companion 2MASS J22410186-4500298 with an estimated spectral type of L2. The primary (LEHPM 5005) is likely a mid-M dwarf but over-luminous for its color, indicating a possible close equal mass binary. According to the Gaia EDR3 parallax of the primary, the system is located at a distance of 58 ± 2 pc. We calculated an angular separation of 7.″2 between both components, resulting in a projected physical separation of 418 au.
2. Second Data Release of the All-sky NOIRLab Source Catalog

   https://doi.org/10.3847/1538-3881/abd6e1  

   Nidever, David L. ; Dey, Arjun ; Fasbender, Katie ; Juneau, Stéphanie ; Meisner, Aaron M. ; Wishart, Joseph ; Scott, Adam ; Matt, Kyle ; Nikutta, Robert ; Pucha, Ragadeepika ( March 2021 , The Astronomical Journal)

   Abstract We announce the second data release (DR2) of the NOIRLab Source Catalog (NSC), using 412,116 public images from CTIO-4 m+DECam, the KPNO-4 m+Mosaic3, and the Bok-2.3 m+90Prime. NSC DR2 contains over 3.9 billion unique objects, 68 billion individual source measurements, covers ≈35,000 square degrees of the sky, has depths of ≈23 mag in most broadband filters with ≈1%–2% photometric precision, and astrometric accuracy of ≈7 mas. Approximately 1.9 billion objects within ≈30,000 square degrees of sky have photometry in three or more bands. There are several improvements over NSC DR1. DR2 includes 156,662 (61%) more exposures extending over 2 more years than in DR1. The southern photometric zero-points in griz are more accurate by using the Skymapper DR1 and ATLAS-Ref2 catalogs, and improved extinction corrections were used for high-extinction regions. In addition, the astrometric accuracy is improved by taking advantage of Gaia DR2 proper motions when calibrating the astrometry of individual images. This improves the NSC proper motions to ∼2.5 mas yr −1 (precision) and ∼0.2 mas yr −1 (accuracy). The combination of sources into unique objects is performed using a DBSCAN algorithm and mean parameters per object (such as mean magnitudes, proper motion, etc.) are calculated more robustlymore »with outlier rejection. Finally, eight multi-band photometric variability indices are calculated for each object and variable objects are flagged (23 million objects). NSC DR2 will be useful for exploring solar system objects, stellar streams, dwarf satellite galaxies, quasi-stellar objects, variable stars, high proper-motion stars, and transients. Several examples of these science use cases are presented. The NSC DR2 catalog is publicly available via the NOIRLab’s Astro Data Lab science platform.« less
3. Astrometric Accelerations as Dynamical Beacons: Discovery and Characterization of HIP 21152 B, the First T-dwarf Companion in the Hyades*

   https://doi.org/10.3847/1538-3881/aca408  

   Franson, Kyle ; Bowler, Brendan P. ; Bonavita, Mariangela ; Brandt, Timothy D. ; Chen, Minghan ; Samland, Matthias ; Zhang, Zhoujian ; Lueber, Anna ; Heng, Kevin ; Kitzmann, Daniel ; et al ( January 2023 , The Astronomical Journal)

   Benchmark brown dwarf companions with well-determined ages and model-independent masses are powerful tools to test substellar evolutionary models and probe the formation of giant planets and brown dwarfs. Here, we report the independent discovery of HIP 21152 B, the first imaged brown dwarf companion in the Hyades, and conduct a comprehensive orbital and atmospheric characterization of the system. HIP 21152 was targeted in an ongoing high-contrast imaging campaign of stars exhibiting proper-motion changes between Hipparcos and Gaia, and was also recently identified by Bonavita et al. (2022) and Kuzuhara et al. (2022). Our Keck/NIRC2 and SCExAO/CHARIS imaging of HIP 21152 revealed a comoving companion at a separation of 0.″37 (16 au). We perform a joint orbit fit of all available relative astrometry and radial velocities together with the Hipparcos-Gaia proper motions, yielding a dynamical mass of${24}_{-4}^{+6}{M}_{\mathrm{Jup}}$, which is 1–2σlower than evolutionary model predictions. Hybrid grids that include the evolution of cloud properties best reproduce the dynamical mass. We also identify a comoving wide-separation (1837″ or 7.9 × 10⁴au) early-L dwarf with an inferred mass near the hydrogen-burning limit. Finally, we analyze the spectra and photometry of HIP 21152 B using the Saumon & Marley (2008)more »atmospheric models and a suite of retrievals. The best-fit grid-based models havef_sed= 2, indicating the presence of clouds,T_eff= 1400 K, and$\log g=4.5\mathrm{dex}$. These results are consistent with the object’s spectral type of T0 ± 1. As the first benchmark brown dwarf companion in the Hyades, HIP 21152 B joins the small but growing number of substellar companions with well-determined ages and dynamical masses.

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4. On the Unusual Variability of 2MASS J06195260–2903592: A Long-lived Disk around a Young Ultracool Dwarf

   https://doi.org/10.3847/1538-3881/ac8cee  

   Liu, Michael C. ; Magnier, Eugene A. ; Zhang, Zhoujian ; Gaidos, Eric ; Dupuy, Trent J. ; Liu, Pengyu ; Biller, Beth A. ; Vos, Johanna M. ; Allers, Katelyn N. ; Hinkle, Jason T. ; et al ( September 2022 , The Astronomical Journal)

   We present the characterization of the low-gravity M6 dwarf 2MASS J06195260-2903592, previously identified as an unusual field object based on its strong IR excess and variable near-IR spectrum. Multiple epochs of low-resolution (R≈ 150) near-IR spectra show large-amplitude (≈0.1–0.5 mag) continuum variations on timescales of days to 12 yr, unlike the small-amplitude variability typical for field ultracool dwarfs. The variations between epochs are well-modeled as changes in the relative extinction (ΔA_V≈ 2 mag). Similarly, Panoramic Survey Telescope and Rapid Response System 1 optical photometry varies on timescales as long as 11 yr (and possibly as short as an hour) and implies comparableA_Vchanges. Near Earth Object Wide-field Infrared Survey Explorer mid-IR light curves also suggest changes on 6 month timescales, with amplitudes consistent with the optical/near-IR extinction variations. However, near-IR spectra, near-IR photometry, and optical photometry obtained in the past year indicate that the source can also be stable on hourly and monthly timescales. From comparison to objects of similar spectral type, the total extinction of 2MASS J0619-2903 seems to beA_V≈ 4–6 mag, with perhaps epochs of lower extinction. Gaia Early Data Release 3 (EDR3) finds that 2MASS J0619-2903 has a wide-separation (1.′2 = 10,450 au) stellar companion, with anmore »isochronal age of${31}_{-10}^{+22}$Myr and a mass of${0.30}_{-0.03}^{+0.04}$M_☉. Adopting this companion’s age and EDR3 distance (145.2 ± 0.6 pc), we estimate a mass of 0.11–0.17M_☉for 2MASS J0619-2903. Altogether, 2MASS J0619-2903 appears to possess an unusually long-lived primordial circumstellar disk, perhaps making it a more obscured analog to the “Peter Pan” disks found around a few M dwarfs in nearby young moving groups.

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5. COol Companions ON Ultrawide orbiTS (COCONUTS). III. A Very Red L6 Benchmark Brown Dwarf around a Young M5 Dwarf

   https://doi.org/10.3847/1538-4357/ac7ce9  

   Zhang, Zhoujian ; Liu, Michael C. ; Morley, Caroline V. ; Magnier, Eugene A. ; Tucker, Michael A. ; Vanderbosch, Zachary P. ; Do, Aaron ; Shappee, Benjamin J. ( August 2022 , The Astrophysical Journal)

   Abstract We present the third discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-3 system, composed of the young M5 primary star UCAC4 374−046899 and the very red L6 dwarf WISEA J081322.19−152203.2. These two objects have a projected separation of 61 ′ ′ (1891 au) and are physically associated given their common proper motions and estimated distances. The primary star, COCONUTS-3A, has a mass of 0.123 ± 0.006 M ⊙ , and we estimate its age as 100 Myr to 1 Gyr based on its stellar activity (via H α and X-ray emission), kinematics, and spectrophotometric properties. We derive its bulk metallicity as 0.21 ± 0.07 dex using empirical calibrations established by older and higher-gravity M dwarfs and find that this [Fe/H] could be slightly underestimated according to PHOENIX models given COCONUTS-3A’s younger age. The companion, COCONUTS-3B, has a near-infrared spectral type of L6 ± 1 int-g , and we infer physical properties of T eff = 1362 − 73 + 48 K, log ( g ) = 4.96 − 0.34 + 0.15 dex, R = 1.03 − 0.06 + 0.12 R Jup , and M = 39 − 18 + 11 M Jup using its bolometricmore »luminosity, its host star’s age, and hot-start evolution models. We construct cloudy atmospheric model spectra at the evolution-based physical parameters and compare them to COCONUTS-3B’s spectrophotometry. We find that this companion possesses ample condensate clouds in its photosphere ( f sed = 1) with the data–model discrepancies likely due to the models using an older version of the opacity database. Compared to field-age L6 dwarfs, COCONUTS-3B has fainter absolute magnitudes and a 120 K cooler T eff . Also, the J − K color of this companion is among the reddest for ultracool benchmarks with ages older than a few hundred megayears. COCONUTS-3 likely formed in the same fashion as stellar binaries given the companion-to-host mass ratio of 0.3 and represents a valuable benchmark to quantify the systematics of substellar model atmospheres.« less