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VHS J1256$-$1257 AB is an ultracool dwarf binary that hosts a wide-separation planetary-mass companion that is a key target of the {\sl JWST} Exoplanet Early Release Science (ERS) program. Using Keck adaptive optics imaging and aperture masking interferometry, we have determined the host binary's orbit, $a=1.96\pm0.03$ au, $P=7.31\pm0.02$ yr, $e=0.883\pm0.003$, and measured its dynamical total mass, $0.141\pm0.008$ $M_{\odot}$. This total mass is consistent with VHS J1256$-$1257 AB being a brown dwarf binary or pair of very low-mass stars. In addition, we measured the orbital motion of VHS J1256$-$1257 b with respect to the barycenter of VHS J1256$-$1257 AB, finding that the wide companion's orbit is also eccentric, $e=0.68^{+0.11}_{-0.10}$, with a mutual inclination of $115\pm14^{\circ}$ with respect to the central binary. This orbital architecture is consistent with VHS J1256$-$1257 b attaining a significant mutual inclination through dynamical scattering and thereafter driving Kozai-Lidov cycles to pump the eccentricity of VHS J1256$-$1257 AB. We derive a cooling age of $140\pm20$ Myr for VHS J1256$-$1257 AB from low-mass stellar/substellar evolutionary models. At this age, the luminosity of VHS J1256$-$1257 b is consistent with both deuterium-inert and deuterium-fusing evolutionary tracks. We thus find a bimodal probability distribution for the mass of VHS J1256$-$1257 b, either $12.0\pm0.1$ $M_{\rm Jup}$ or $16\pm1$ $M_{\rm Jup}$, from these models. Future spectroscopic data to measure isotopologues such as HDO and CH$_3$D could break this degeneracy and provide a strong test of substellar models at the deuterium-fusion mass boundary., MNRAS published version; MCMC posteriors now included in source material; AB-b orbital parameters are updated after correcting a typo in the Dec orbital motion of b

We present the third discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-3 system, composed of a young M5 primary star UCAC4 374-046899 and a 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 $0.123\pm0.006$ M$_{\odot}$ and we estimate its age as 100 Myr to 1 Gyr based on its kinematics and spectrophotometric properties. We derive its metallicity as $0.21 \pm 0.07$ dex using empirical calibrations established by older higher-gravity M dwarfs and find 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$\pm$1 INT-G, and we infer physical properties of $T_{\rm eff} = 1362^{+48}_{-73}$ K, $\log{(g)}= 4.96^{+0.15}_{-0.34}$ dex, $R = 1.03^{+0.12}_{-0.06}$ R$_{\rm Jup}$, and $M = 39^{+11}_{-18}$ M$_{\rm Jup}$, using its bolometric 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 this companion possesses ample condensate clouds in its photosphere 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 $T_{\rm eff}$. Also, the J-K color of this companion is among the reddest for ultracool benchmarks with ages older than a few 100 Myr. 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., Accepted by ApJ. 34 pages including 14 figures and 5 tables

Full list of authors: Hinkley, Sasha; Carter, Aarynn L.; Ray, Shrishmoy; Skemer, Andrew; Biller, Beth; Choquet, Elodie; Millar-Blanchaer, Maxwell A.; Sallum, Stephanie; Miles, Brittany; Whiteford, Niall; Patapis, Polychronis; Perrin, Marshall; Pueyo, Laurent; Schneider, Glenn; Stapelfeldt, Karl; Wang, Jason; Ward-Duong, Kimberly; Bowler, Brendan P.; Boccaletti, Anthony; Girard, Julien H.; Hines, Dean; Kalas, Paul; Kammerer, Jens; Kervella, Pierre; Leisenring, Jarron; Pantin, Eric; Zhou, Yifan; Meyer, Michael; Liu, Michael C.; Bonnefoy, Mickael; Currie, Thayne; McElwain, Michael; Metchev, Stanimir; Wyatt, Mark; Absil, Olivier; Adams, Jea; Barman, Travis; Baraffe, Isabelle; Bonavita, Mariangela; Booth, Mark; Bryan, Marta; Chauvin, Gael; Chen, Christine; Danielski, Camilla; De Furio, Matthew; Factor, Samuel M.; Fitzgerald, Michael P.; Fortney, Jonathan J.; Grady, Carol; Greenbaum, Alexandra; Henning, Thomas; Hoch, Kielan K. W.; Janson, Markus; Kennedy, Grant; Kenworthy, Matthew; Kraus, Adam; Kuzuhara, Masayuki; Lagage, Pierre-Olivier; Lagrange, Anne-Marie; Launhardt, Ralf; Lazzoni, Cecilia; Lloyd, James; Marino, Sebastian; Marley, Mark; Martinez, Raquel; Marois, Christian; Matthews, Brenda; Matthews, Elisabeth C.; Mawet, Dimitri; Mazoyer, Johan; Phillips, Mark; Petrus, Simon; Quanz, Sascha P.; Quirrenbach, Andreas; Rameau, Julien; Rebollido, Isabel; Rickman, Emily; Samland, Matthias; Sargent, B.; Schlieder, Joshua E.; Sivaramakrishnan, Anand; Stone, Jordan M.; Tamura, Motohide; Tremblin, Pascal; Uyama, Taichi; Vasist, Malavika; Vigan, Arthur; Wagner, Kevin; Ygouf, Marie.--This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/3.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited., The direct characterization of exoplanetary systems with high-contrast imaging is among the highest priorities for the broader exoplanet community. As large space missions will be necessary for detecting and characterizing exo-Earth twins, developing the techniques and technology for direct imaging of exoplanets is a driving focus for the community. For the first time, JWST will directly observe extrasolar planets at mid-infrared wavelengths beyond 5 μm, deliver detailed spectroscopy revealing much more precise chemical abundances and atmospheric conditions, and provide sensitivity to analogs of our solar system ice-giant planets at wide orbital separations, an entirely new class of exoplanet. However, in order to maximize the scientific output over the lifetime of the mission, an exquisite understanding of the instrumental performance of JWST is needed as early in the mission as possible. In this paper, we describe our 55 hr Early Release Science Program that will utilize all four JWST instruments to extend the characterization of planetary-mass companions to ∼15 μm as well as image a circumstellar disk in the mid-infrared with unprecedented sensitivity. Our program will also assess the performance of the observatory in the key modes expected to be commonly used for exoplanet direct imaging and spectroscopy, optimize data calibration and processing, and generate representative data sets that will enable a broad user base to effectively plan for general observing programs in future Cycles. © 2022. The Author(s). Published by IOP Publishing Ltd on behalf of the Astronomical Society of the Pacific (ASP). All rights reserved., This project was supported by a grant from STScI (JWST-ERS-01386) under NASA contract NAS5-03127. M.T.is supported by JSPS KAKENHI Grant No.18H05442. M.B. acknowledges support from the Deutsche Forschungsgemeinschaft (DFG) through grant Kr 2164/13–2. K.W. acknowledges support from NASA through the NASA Hubble Fellowship grant HST-HF2-51472.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

We present the highest fidelity spectrum to date of a planetary-mass object. VHS 1256 b is a $, Comment: Accepted ApJL Iterations of spectra reduced by the ERS team are hosted at this link: https://github.com/bemiles/JWST_VHS1256b_Reduction/tree/main/reduced_spectra

We report the discovery of substellar objects in the young star cluster IC 348 and the neighboring Barnard 5 dark cloud, both at the eastern end of the Perseus star-forming complex. The substellar candidates are selected using narrowband imaging, i.e., on and off photometric technique with a filter centered around the water absorption feature at 1.45 microns, a technique proven to be efficient in detecting water-bearing substellar objects. Our spectroscopic observations confirm three brown dwarfs in IC 348. In addition, the source WBIS 03492858+3258064, reported in this work, is the first confirmed brown dwarf discovered toward Barnard 5. Together with the young stellar population selected via near- and mid-infrared colors using the Two Micron All Sky Survey and the Wide-field Infrared Survey Explorer, we diagnose the relation between stellar versus substellar objects with the associated molecular clouds. Analyzed by Gaia EDR3 parallaxes and kinematics of the cloud members across the Perseus region, we propose the star formation scenario of the complex under influence of the nearby OB association., Comment: Accepted for publication in the Astrophysical Journal. 24 pages, 14 figures, 5 tables

Using a volume-limited sample of 263 field-age brown dwarfs defined by parallaxes, we provide updated polynomials for CatWISE W1 and W2 absolute magnitudes as a function of spectral type. Our fits are consistent with previous relations and apply to a wider M6–Y1 spectral type range.

Advanced techniques of evaluating cumulative material damage for power plant components operating in the creep range can enhance life management strategies, especially as power plants transition to more flexible operations. Power piping creep damage is strongly sensitive to the operating temperatures, operating pressures, and applied stresses. Several empirical examples reveal large ranges of operating temperatures and pressures throughout the operating years of power piping systems. Multiple examples reveal that the operating conditions may be significantly different over the years. Time and pressure histories for main steam (MS) and hot reheat (HRH) piping systems are provided to illustrate these operational changes, including mean and median annual and monthly plant information data. Operating temperature and pressure histograms can reveal single modal and bi-modal plant operations. Examples of mean, median, and upper 95% confidence operating temperature and pressure values are included in this study. There may be some instances of operating above design temperatures and design pressures for short periods of time. Advanced evaluations have revealed that some above design temperature events have occurred with low operating pressures, so coincident operating pressure and operating temperature evaluations are also discussed to determine more accurate estimates of cumulative creep damage. Accelerated creep-fatigue damage can occur from high-temperature ramp rates obtained from the detailed temperature profiles. Evaluation of the applicable applied stresses in piping system weldments should consider the simulation as-found stresses, redistributed through-wall axial and circumferential stresses, weld residual stresses, and redistributed multiaxial stresses. The hot and cold piping system walkdowns should compare the actual piping displacements to the predicted design displacements. Piping walkdown evaluations should identify and evaluate the observed significant field anomalies. Several empirical examples illustrate that the simulation as-found stress analysis (considering adverse field conditions) can result in piping stresses significantly different than those predicted in the as-designed piping stress analysis. In the past, the selection of weldment reexamination intervals has been based on standard industry practice. Advanced predictive analytic evaluations can be used to provide a technical justification for continued plant operation until critical components are reexamined. This study provides several examples of using the actual operating temperatures, operating pressures, field stresses, and cumulative operating hours to estimate the cumulative creep/fatigue damage at critical system components. Assuming future piping system operations the same as in the past, remaining useful lives (RULs) for the critical system components can be estimated. The piping system RUL contour isometric illustrates the most critical girth weldment creep damage rankings (these are the lead-the-fleet nondestructive examination (NDE) locations). If there are no significant fabrication defects in the other weldments (which can be confirmed by a one-time examination), the evaluation can be used to justify that many of the remaining weldments do not have to be examined for more than a decade if future operation is the same as in the past.

The spectra of brown dwarfs are key to exploring the chemistry and physics that take place in their atmospheres. Late-T dwarf spectra are particularly diagnostic due to their relatively cloud-free atmospheres and deep molecular bands. With the use of powerful atmospheric retrieval tools applied to the spectra of these objects, direct constraints on molecular/atomic abundances, gravity, and vertical thermal profiles can be obtained enabling a broad exploration of the chemical/physical mechanisms operating in their atmospheres. We present a uniform retrieval analysis on low-resolution IRTF SpeX near-IR spectra of a sample of 50 T dwarfs, including new observations as part of a recent volume-limited survey. This analysis more than quadruples the sample of T dwarfs with retrieved temperature profiles and abundances (H$_2$O, CH$_4$, NH$_3$, K and subsequent C/O and metallicities). We are generally able to constrain effective temperatures to within 50K, volume mixing ratios for major species to within 0.25dex, atmospheric metallicities [M/H] to within 0.2, and C/O ratios to within 0.2. We compare our retrieved constraints on the thermal structure, chemistry, and gravities of these objects with predictions from self-consistent radiative-convective equilibrium models and find, in general though with substantial scatter, consistency with solar composition chemistry and thermal profiles of the neighboring stellar FGK population. Objects with notable discrepancies between the two modeling techniques and potential mechanisms for their differences, be they related to modeling approach or physically motivated, are discussed more thoroughly in the text., 21 pages, 11 figures

The binary nature of the M8.5 dwarf DENIS J063001.4$-$184014AB (DE0630$-$18) was discovered with astrometric monitoring from the ground, which determined the unresolved photocentric orbit and the trigonometric parallax of the system. Here we present radial-velocity monitoring and resolved observations in the near-infrared with Keck aperture masking that allow us to measure the system's relative separation and brightness. By combining all available information, we determine the individual dynamical masses of the binary components to be $M_1 = 0.052^{+0.009}_{-0.008} M_\mathrm{Sun}$ and $M_2 = 0.052^{+0.005}_{-0.004} M_\mathrm{Sun}$, both firmly in the substellar regime. These masses are surprising given the object's M8.5 optical spectral type and equivalent absolute magnitude, and the significant difference in brightness between the components ($\Delta{K}$ = 1.74$\pm$0.06 mag). Our results suggest that DE0630$-$18 is a relatively young system ($\sim$200 Myr) with a secondary component that is itself a potentially unresolved binary., Comment: 10 pages, 8 figures, accepted for publication in MNRAS

We present CFHT photometry and IRTF spectroscopy of low-mass candidate members of Serpens South and Serpens Core ($\sim$430 pc, $\sim$0.5 Myr), identified using a novel combination of photometric filters, known as the W-band method. We report SC182952+011618, SS182959-020335 and SS183032-021028 as young, low-mass Serpens candidate members, with spectral types in the range M7-M8, M5-L0 and M5-M6.5 respectively. Best-fit effective temperatures and luminosities imply masses of $, Comment: Accepted for publication in MNRAS

We report the discovery of a resolved (0".9) substellar companion to a member of the 1-5 Myr Taurus star-forming region. The host star (2M0437) is a single mid-M type ($T_{eff}\approx$3100K) dwarf with a position, space motion, and color-magnitude that support Taurus membership, and possible affiliation with a $\sim$2.5 Myr-old sub-group. A comparison with stellar models suggests a 2-5 Myr age and a mass of 0.15-0.18M$_{\odot}$. Although K2 detected quasi-periodic dimming from close-in circumstellar dust, the star lacks detectable excess infrared emission from a circumstellar disk and its H$\alpha$ emission is not commensurate with accretion. Astrometry based on three years of AO imaging shows that the companion (2M0437b) is co-moving, while photometry of two other sources at larger separation indicates they are likely heavily-reddened background stars. A comparison of the luminosity of 2M0437b with models suggests a mass of 3-5$M_{\rm jup}$, well below the deuterium burning limit, and an effective temperature of 1400-1500K, characteristic of a late L spectral type. The $H$-$K$ color is redder than the typical L dwarf, but comparable to other directly detected young planets, e.g. those around HR 8799. The discovery of a super-Jupiter around a very young, very low mass star challenges models of planet formation by either core accretion (which requires time) or disk instability (which requires mass). We also detected a second, co-moving, widely-separated (75") object which appears to be a heavily-extincted star. This is certainly a fellow member of this Taurus sub-group and statistically likely to be a bound companion., Comment: Accepted to MNRAS 14 October 2021

We present comprehensive orbital analyses and dynamical masses for the substellar companions Gl~229~B, Gl~758~B, HD~13724~B, HD~19467~B, HD~33632~Ab, and HD~72946~B. Our dynamical fits incorporate radial velocities, relative astrometry, and most importantly calibrated Hipparcos-Gaia EDR3 accelerations. For HD~33632~A and HD~72946 we perform three-body fits that account for their outer stellar companions. We present new relative astrometry of Gl~229~B with Keck/NIRC2, extending its observed baseline to 25 years. We obtain a $, Comment: Accepted for publication in AJ. References updated in version 2. See the journal version for the full quality figures. Figure sets and the MCMC chains (reduced to just 1000 samples however) are included with the journal version of the article, and pre-publication at https://drive.google.com/drive/folders/1_A8QYn9NyPgmGqJaY5sMHyT_wAS3uRRK?usp=sharing

We present results from the Large Adaptive optics Survey for Substellar Objects (LASSO), where the goal is to directly image new substellar companions (11.8$), while only 23$^{+11}_{-6}$% of the remaining 22 stars with no detected companion have significant accelerations. The significance of the acceleration decreases with increasing companion separation. These young accelerating low-mass stars with companions will eventually yield dynamical masses with future orbit monitoring., 44 pages: 20 pages of text with 14 figures, 24 pages of tables in the appendix. Accepted for publication in the Astronomical Journal

We present a large forward-modeling analysis for 55 late-T (T7-T9) dwarfs, using low-resolution ($R\approx150$) near-infrared spectra and cloudless Sonora-Bobcat model atmospheres. We derive the objects' effective temperatures, surface gravities, metallicities, radii, masses, and luminosities using our newly developed Bayesian framework, and use the resulting population properties to test the model atmospheres. We find (1) our objects' fitted metallicities are 0.3-0.4 dex lower than those of nearby stars; (2) their ages derived from spectroscopic parameters are implausibly young; (3) their fitted temperatures show a similar spread as empirical temperature scales at a given spectral type but are $\sim100$ K hotter for $\geqslant$T8 dwarfs; and (4) their spectroscopically inferred masses are unphysically small. These results suggest the Sonora-Bobcat assumptions of cloudless and chemical-equilibrium atmospheres do not adequately reproduce late-T dwarf spectra. We also find a gravity- and a metallicity-dependence of temperatures. Combining the resulting parameter posteriors of our sample, we quantify the degeneracy between surface gravity and metallicity such that an increase in $Z$ combined with a $3.4\times$ increase in $\log{g}$ results in a spectrum that has similar fitted parameters. We note the systematic difference between our 1.0-2.5 $\mu$m spectra and the Sonora-Bobcat models is $\approx$2-4% of the objects' peak $J$-band fluxes, implying modeling systematics will exceed measurement uncertainties when analyzing data with $J$-band S/N $\gtrsim50$. Using our large sample, we examine the fitting residuals as a function of wavelength and atmospheric properties to discern how to improve the models. Our work constitutes the largest analysis of brown dwarf spectra using multi-metallicity models and the most systematic examination of ultracool model atmospheres to date., Comment: Accepted by ApJ. 52 pages including 21 figures and 9 tables

We present a search for new planetary-mass members of nearby young moving groups (YMGs) using astrometry for 694 T and Y dwarfs, including 447 objects with parallaxes, mostly produced by recent large parallax programs from UKIRT and Spitzer. Using the BANYAN $\Sigma$ and LACEwING algorithms, we identify 30 new candidate YMG members, with spectral types of T0$-$T9 and distances of $10-43$ pc. Some candidates have unusually red colors and/or faint absolute magnitudes compared to field dwarfs with similar spectral types, providing supporting evidence for their youth, including 4 early-T dwarfs. We establish one of these, the variable T1.5 dwarf 2MASS J21392676$+$0220226, as a new planetary-mass member ($14.6^{+3.2}_{-1.6}$ M$_{\rm Jup}$) of the Carina-Near group ($200\pm50$ Myr) based on its full six-dimensional kinematics, including a new parallax measurement from CFHT. The high-amplitude variability of this object is suggestive of a young age, given the coexistence of variability and youth seen in previously known YMG T dwarfs. Our four latest-type (T8$-$T9) YMG candidates, WISE J031624.35$+$430709.1, ULAS J130217.21$+$130851.2, WISEPC J225540.74$-$311841.8, and WISE J233226.49$-$432510.6, if confirmed, will be the first free-floating planets ($\approx2-6$ M$_{\rm Jup}$) whose ages and luminosities are compatible with both hot-start and cold-start evolutionary models, and thus overlap the properties of the directly-imaged planet 51 Eri b. Several of our early/mid-T candidates have peculiar near-infrared spectra, indicative of heterogenous photospheres or unresolved binarity. Radial velocity measurements needed for final membership assessment for most of our candidates await upcoming 20$-$30 meter class telescopes. In addition, we compile all 15 known T7$-$Y1 benchmarks and derive a homogeneous set of their effective temperatures, surface gravities, radii, and masses., Comment: ApJ, in press. 27 pages including 6 figures and 5 tables

We present the discovery of a planetary-mass companion to CFHTWIR-Oph 98, a low-mass brown dwarf member of the young Ophiuchus star-forming region, with a wide 200-au separation (1.46 arcsec). The companion was identified using Hubble Space Telescope images, and confirmed to share common proper motion with the primary using archival and new ground-based observations. Based on the very low probability of the components being unrelated Ophiuchus members, we conclude that Oph 98 AB forms a binary system. From our multi-band photometry, we constrain the primary to be an M9-L1 dwarf, and the faint companion to have an L2-L6 spectral type. For a median age of 3 Myr for Ophiuchus, fits of evolutionary models to measured luminosities yield masses of $15.4\pm0.8$ M$_\mathrm{Jup}$ for Oph 98 A and $7.8\pm0.8$ M$_\mathrm{Jup}$ for Oph 98 B, with respective effective temperatures of $2320\pm40$ K and $1800\pm40$ K. For possible system ages of 1-7 Myr, masses could range from 9.6-18.4 M$_\mathrm{Jup}$ for the primary, and from 4.1-11.6 M$_\mathrm{Jup}$ for the secondary. The low component masses and very large separation make this binary the lowest binding energy system imaged to date, indicating that the outcome of low-mass star formation can result in such extreme, weakly-bound systems. With such a young age, Oph 98 AB extends the growing population of young free-floating planetary-mass objects, offering a new benchmark to refine formation theories at the lowest masses., Comment: Accepted for publication in ApJL. 4 figures, 2 tables

We present the status and plans for the Keck All sky Precision Adaptive optics (KAPA) program. KAPA includes four key science programs, an upgrade to the Keck I laser guide star (LGS) adaptive optics (AO) facility to improve image quality and sky coverage, AO telemetry based point spread function (PSF) estimates for all science exposures, and an educational component focused on broadening the participation of women and underrepresented groups in instrumentation. For the purpose of this conference we will focus on the AO facility upgrade which includes implementation of a new laser, wavefront sensor and real-time controller to support laser tomography, the laser tomography system itself, and modifications to an existing near-infrared tip-tilt sensor to support multiple natural guide star (NGS) and focus measurements.

A benchmark brown dwarf (BD) is a BD whose properties (e.g., mass and chemical composition) are precisely and independently measured. Benchmark BDs are valuable in testing theoretical evolutionary tracks, spectral synthesis, and atmospheric retrievals for sub-stellar objects. Here, we report results of atmospheric retrieval on a synthetic spectrum and a benchmark BD -- HR 7672~B -- with \petit. First, we test the retrieval framework on a synthetic PHOENIX BT-Settl spectrum with a solar composition. We show that the retrieved C and O abundances are consistent with solar values, but the retrieved C/O is overestimated by 0.13-0.18, which is $\sim$4 times higher than the formal error bar. Second, we perform retrieval on HR 7672~B using high spectral resolution data (R=35,000) from the Keck Planet Imager and Characterizer (KPIC) and near infrared photometry. We retrieve [C/H], [O/H], and C/O to be $-0.24\pm0.05$, $-0.19\pm0.04$, and $0.52\pm0.02$. These values are consistent with those of HR 7672~A within 1.5-$\sigma$. As such, HR 7672~B is among only a few benchmark BDs (along with Gl 570~D and HD 3651~B) that have been demonstrated to have consistent elemental abundances with their primary stars. Our work provides a practical procedure of testing and performing atmospheric retrieval, and sheds light on potential systematics of future retrievals using high- and low-resolution data., Comment: 29 pages, 17 figures, 5 tables, resubmitted to AAS journals after first revision

We present a forward-modeling framework using the Bayesian inference tool Starfish and cloudless Sonora-Bobcat model atmospheres to analyze low-resolution ($R\approx80-250$) near-infrared ($1.0-2.5$ $\mu$m) spectra of T dwarfs. Our approach infers effective temperatures, surface gravities, metallicities, radii, and masses, and by accounting for uncertainties from model interpolation and correlated residuals due to instrumental effects and modeling systematics, produces more realistic parameter posteriors than traditional ($\chi^2$-based) spectral-fitting analyses. We validate our framework by fitting the model atmospheres themselves and finding negligible offsets between derived and input parameters. We apply our methodology to three well-known benchmark late-T dwarfs, HD 3651B, GJ 570D, and Ross 458C, using both solar and non-solar metallicity atmospheric models. We also derive these benchmarks' physical properties using their bolometric luminosities, their primary stars' ages and metallicities, and Sonora-Bobcat evolutionary models. Assuming the evolutionary-based parameters are more robust, we find our atmospheric-based, forward-modeling analysis produces two outcomes. For HD 3615B and GJ 570D, spectral fits provide accurate $T_{\rm eff}$ and $R$ but underestimated $\log{g}$ (by $\approx1.2$ dex) and $Z$ (by $\approx0.35$ dex), likely due to the systematics from modeling the potassium line profiles. For Ross 458C, spectral fits provide accurate $\log{g}$ and $Z$ but overestimated $T_{\rm eff}$ (by $\approx120$ K) and underestimated $R$ (by $\approx1.6\times$), likely because our model atmospheres lack clouds, reduced vertical temperature gradients, or disequilibrium processes. Finally, the spectroscopically inferred masses of these benchmarks are all considerably underestimated., Comment: Accepted by ApJ. Minor updates since v1: (1) calculation of (very small) effect by different photometric calibrations (Sec 4.1.6), (2) expanded literature review about possible systematics in evolutionary models (Sec 6), (3) examination of the (very small) difference between Sonora vs ATMO models for luminosity calculations (footnote 10), (4) slight revision to calculation plotted in Figure 2

Magnetospheric processes seen in gas-giants such as aurorae and circularly-polarized cyclotron maser radio emission have been detected from some brown dwarfs. However, previous radio observations targeted known brown dwarfs discovered via their infrared emission. Here we report the discovery of BDR J1750+3809, a circularly polarized radio source detected around 144 MHz with the LOFAR telescope. Follow-up near-infrared photometry and spectroscopy show that BDR J1750+3809 is a cold methane dwarf of spectral type T$6.5\pm 1$ at a distance of $65^{+9}_{-8}\,{\rm pc}$. The quasi-quiescent radio spectral luminosity of BDR J1750+3809 is $\approx 5\times 10^{15}\,{\rm erg}\,{\rm s}^{-1}\,{\rm Hz}^{-1}$ which is over two orders of magnitude larger than that of the known population of comparable spectral type. This could be due to a preferential geometric alignment or an electrodynamic interaction with a close companion. In addition, as the emission is expected to occur close to the electron gyro-frequency, the magnetic field strength at the emitter site in BDR J1750+3809 is $B\gtrsim 25\,{\rm G}$, which is comparable to planetary-scale magnetic fields. Our discovery suggests that low-frequency radio surveys can be employed to discover sub-stellar objects that are too cold to be detected in infrared surveys., Comment: Accepted for publication in ApJL

Spectra of brown dwarfs are key to exploring the chemistry and physics that take place in their atmospheres. Late T dwarf (850 - 500 K) spectra are particularly diagnostic due to their relatively cloud free atmospheres and deep molecular bands. With the use of powerful atmospheric retrieval tools, these properties permit proper allocation of constraints on the molecular/atomic abundances and temperature profiles. These constraints can be used to derive the elemental abundances (metallicity, C/O), chemical disequilibrium, and non-radiative-convective equilibrium temperature perturbations. Building upon previous analyses on T and Y dwarfs (Line et al. 2017; Zalesky et al. 2019), we present a uniform retrieval analysis of 52 T dwarfs via their low-resolution near-infrared spectra. This analysis more than doubles the sample of T dwarfs with retrieved properties. We present updates on current compositional trends and thermal profile constraints amongst the T dwarf population.

Until the 2000 Edition of API-579 [1], the geometries (Ri/t, a/ℓ, a/t in cylinders or spheres) for which a complete set of stress intensity factor influence coefficients (Gi) existed was limited. To bridge that gap, many analysts have used the G0 and G1 coefficients for flat plate geometries to represent axial or circumferential flaws in cylinders. Because the cylindrical geometry contains additional stiffness effects due to curvature, it was generally assumed that the plate geometry (Ri/t = ∞) will be bounding and therefore conservative [2]. Since 2007, extensive sets of Gi coefficient have been published for plates, cylinders and spheres based on finite element models. These coefficients for cylinders have been curve fit and are available in Appendix A of the ASME Boiler and Pressure Vessel Code, Section XI [3]. This paper compares the G0 coefficients at the deepest point of semi-elliptical flaws from a plate to those for an OD and ID circumferential flaw in a cylinder to confirm the conservatism of the G0 coefficients from a plate geometry.

We present the design and implementation of a medium-band near-IR filter tailored for detecting low-mass stars and brown dwarfs from the summit of Maunakea. The W-band filter is centered at 1.45 micron with a bandpass width of 6%, designed to measure the depth of the H2O water absorption prominent in objects with spectral types of M6 and later. When combined with standard J- and H-band photometry, the W-band filter is designed to determine spectral types to $\approx$1.4 subtypes for late-M and L dwarfs, largely independent of surface gravity and reddening. This filter's primary application is completing the census of young substellar objects in star-forming regions, using W-band selection to greatly reduce contamination by reddened background stars that impede broad-band imaging surveys. We deployed the filter on the UH 88-inch telescope to survey $\approx$3 sq. deg. of the NGC 1333, IC 348, and rho Ophiuchus star-forming regions. Our spectroscopic followup of W-band selected candidates resulted in the confirmation of 48 ultracool dwarfs with a success rate of 89%, demonstrating the efficacy of this new filter and selection method., PASP, in press

We present \textit{Spitzer Space Telescope} variability monitoring observations of three low-gravity L dwarfs with previous detections of variability in the near-IR, 2MASS J0045+16, 2MASS J0501-00 and 2MASS J1425-36. We detect significant, periodic variability in two of our targets, 2MASS J0045+16 and 2MASS J0501-00. We do not detect variability in 2MASS J1425-36. Combining our new rotation periods with rotational velocities, we calculate inclination angles of $22\pm1^{\circ}$, ${60^{+13 }_{-8}} ^{\circ}$ and $52^{+19}_{-13}~^{\circ}$ for 2MASS J0045+16, 2MASS J0501-00 and 2MASS J1425-36 respectively. Our three new objects are consistent with the tentative relations between inclination, amplitude and color anomaly previously reported. Objects with the highest variability amplitudes are inclined equator-on, while the maximum observed amplitude decreases as the inclination angle decreases. We also find a correlation between the inclination angle and $(J-K)_{\mathrm{2MASS}}$ color anomaly for the sample of objects with measured inclinations. Compiling the entire sample of brown dwarfs with \textit{Spitzer} variability detections, we find no enhancement in amplitude for young, early-L dwarfs compared to the field dwarf population. We find a possible enhancement in amplitude of low-gravity late-L dwarfs at $4.5~\mu$m. We do not find a correlation between amplitude ratio and spectral type for field dwarfs or for the young population. Finally, we compile the rotation periods of a large sample of brown dwarfs with ages 1 Myr to 1 Gyr and compare the rotation rates predicted by evolutionary models assuming angular momentum conservation. We find that the rotation rates of the current sample of brown dwarfs fall within the expected range set by evolutionary models and breakup limits., Comment: Accepted for publication in AJ

We present a detailed characterization of 2MASS J04435750+3723031, a low-mass companion orbiting the young M2 star, 2MASS J04435686+3723033, at 7.6 arcseconds (550 AU) with potential membership in the 23 Myr $\beta$ Pictoris moving group ($\beta$PMG). Using near-infrared spectroscopy of the companion from IRTF/SpeX we have found a spectral type of M6 $\pm$ 1 and indications of youth through age-sensitive absorption lines and a low surface gravity index (VL-G). A young age is supported by H$\alpha$ emission and lithium absorption in the host. We re-evaluate the membership of this system and find that it is a marginally consistent kinematic match to the $\beta$PMG using $Gaia$ parallaxes and new radial velocities for the host and companion. If this system does belong to the $\beta$PMG, it would be a kinematic outlier and the companion would be over-luminous compared to other similar ultracool objects like PZ Tel B; this would suggest 2M0443+3723 B could be a close brown dwarf binary ($\approx$52+52 M$_\mathrm{Jup}$ if equal-flux, compared with 99 $\pm$ 5 M$_\mathrm{Jup}$ if single), and would make it the sixth substellar companion in this group. To test this hypothesis, we acquired NIR AO images with Keck II/NIRC2, but they do not resolve the companion to be a binary down to the diffraction limit of $\sim$3 AU. If 2M0443+3723 AB does not belong to any moving group then its age is more uncertain. In this case it is still young ($\lesssim$30 Myr), and the implied mass of the companion would be between $\sim$30--110 M$_\mathrm{Jup}$., Comment: 18 pages, 18 figures, Accepted to ApJ

Given its relative proximity (~430 pc), compact size (< 20'), young age (~ 0.5 Myr) and rich number of young stellar objects, the Serpens-South star forming region is a promising site for studying young sub-stellar objects, yet the low-mass members of this region remain largely undiscovered. In this paper we report on a deep photometric survey using a custom 1.45 um filter (W-band), as well as standard J and H near-IR filters, in order to identify candidate low-mass young brown-dwarfs in the Serpens-South region. We constructed a reddening-insensitive index (Q) by combining J, H and W-band photometry for survey objects, in order to identify candidate low-mass members of Serpens based on the strength of the water absorption feature at 1.45 um in the atmospheres of mid-M and later objects. We then conducted spectroscopic follow up to confirm youth and spectral type for our candidates. This is the first survey to identify the very low-mass and coolest members of Serpens-South. We identify 4 low-mass candidate Serpens members, which all display IR excess emission, indicating the likely presence of circumstellar disks around them. One of the four candidate low-mass members in our list, SERP182918-020245, exhibits Pa_beta and Br_gamma emission features, confirming its youth and ongoing magnetospheric accretion. Our new candidate members have spectral types >M4 and are the coolest and lowest mass candidate members yet identified in Serpens-South., Published in ApJ, 23 pages, 12 figures