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Abstract Most brown dwarfs show some level of photometric or spectral variability. However, finding the most variable dwarfs more suited for a thorough variability monitoring campaign remained a challenge until a few years ago with the design of spectral indices to find the most likely L and T dwarfs using their near-infrared (NIR) single-epoch spectrum. In this work, we designed and tested NIR spectral indices to preselect the most likely variable L4–L8 dwarfs, complementing the indices presented by Ashraf et al. and Oliveros-Gomez et al. We used time-resolved NIR Hubble Space Telescope Wide Field Camera 3 spectra of an L6.0 dwarf, LP 261–75b, to design our novel spectral indices. We tested these spectral indices on 75 L4.0–L8.0 NIR SpeX/IRTF spectra, providing 27 new variable candidates. Our indices have a recovery rate of ∼80% and a false negative rate of ∼25%. All the known nonvariable brown dwarfs were found to be nonvariable by our indices. We estimated the variability fraction of our sample to be ${51}_{-38}^{+4}$%, which agrees with the variability fractions provided by Buenzli et al., Radigan et al., and Metchev et al. for L4–L8 dwarfs. These spectral indices may support the future selection of the most likely variable directly imaged exoplanets for studies with the James Webb Space Telescope and as well as the 30 m telescopes. 

Abstract We present an atmospheric retrieval analysis on a set of young, cloudy, red L dwarfs—CWISER J124332.12+600126.2 (BD+60 1417B) and WISEP J004701.06+680352.1 (W0047)—using the Brewster retrieval framework. We also present the first elemental abundance measurements of the young K-dwarf (K0) host star, BD+60 1417, using high-resolution (R= 50,000) spectra taken with the Potsdam Echelle Polarimetric and Spectroscopic Instrument on the Large Binocular Telescope. In the complex cloudy L-dwarf regime the emergence of condensate cloud species complicates retrieval analysis when only near-infrared data are available. We find that for both L dwarfs in this work, despite testing three different thermal profile parameterizations we are unable to constrain reliable abundance measurements and thus the carbon-to-oxygen ratio. While we cannot conclude what the abundances are, we can conclude that the data strongly favor a cloud model over a cloudless model. We note that the difficulty in retrieval constraints persists regardless of the signal-to-noise ratio of the data examined (S/N ∼ 10 for CWISER BD+60 1417B and 40 for WISEP W0047). The results presented in this work provide valuable lessons about retrieving young, low-surface-gravity cloudy L dwarfs. This work provides continued evidence of missing information in models and the crucial need for JWST to guide and inform retrieval analysis in this regime. 

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. 

Abstract We report direct observational evidence for a latitudinal dependence of dust cloud opacity in ultracool dwarfs, indicating that equatorial latitudes are cloudier than polar latitudes. These results are based on a strong positive correlation between the viewing geometry and the mid-infrared silicate absorption strength in mid-L dwarfs using mid-infrared spectra from the Spitzer Space Telescope and spin axis inclination measurements from available information in the literature. We confirmed that the infrared color anomalies of L dwarfs positively correlate with dust cloud opacity and viewing geometry, where redder objects are inclined equator-on and exhibit more opaque dust clouds, while dwarfs viewed at higher latitudes and with more transparent clouds are bluer. These results show the relevance of viewing geometry to explain the appearance of brown dwarfs and provide insight into the spectral diversity observed in substellar and planetary atmospheres. We also find a hint that dust clouds at similar latitudes may have higher opacity in low-surface gravity dwarfs than in higher-gravity objects. 

Abstract Beyond our Solar System, aurorae have been inferred from radio observations of isolated brown dwarfs^1,2. Within our Solar System, giant planets have auroral emission with signatures across the electromagnetic spectrum including infrared emission of H₃⁺and methane. Isolated brown dwarfs with auroral signatures in the radio have been searched for corresponding infrared features, but only null detections have been reported³. CWISEP J193518.59-154620.3. (W1935 for short) is an isolated brown dwarf with a temperature of approximately 482 K. Here we report James Webb Space Telescope observations of strong methane emission from W1935 at 3.326 μm. Atmospheric modelling leads us to conclude that a temperature inversion of approximately 300 K centred at 1–10 mbar replicates the feature. This represents an atmospheric temperature inversion for a Jupiter-like atmosphere without irradiation from a host star. A plausible explanation for the strong inversion is heating by auroral processes, although other internal and external dynamical processes cannot be ruled out. The best-fitting model rules out the contribution of H₃⁺emission, which is prominent in Solar System gas giants. However, this is consistent with rapid destruction of H₃⁺at the higher pressure where the W1935 emission originates⁴. 

Abstract The Tucana–Horologium association (Tuc-Hor) is a 40 Myr old moving group in the southern sky. In this work, we measure the rotation periods of 313 Tuc-Hor objects with TESS light curves derived from TESS full-frame images and membership lists driven by Gaia EDR3 kinematics and known youth indicators. We recover a period for 81.4% of the sample and report 255 rotation periods for Tuc-Hor objects. From these objects we identify 11 candidate binaries based on multiple periodic signals or outlier Gaia DR2 and EDR3 renormalized unit weight error values. We also identify three new complex rotators (rapidly rotating M dwarf objects with intricate light-curve morphology) within our sample. Along with the six previously known complex rotators that belong to Tuc-Hor, we compare their light-curve morphology between TESS Cycle 1 and Cycle 3 and find that they change substantially. Furthermore, we provide context for the entire Tuc-Hor rotation sample by describing the rotation period distributions alongside other youth indicators such as Hα and Li equivalent width, as well as near-ultraviolet and X-ray flux. We find that measuring rotation periods with TESS is a fast and effective means to confirm members in young moving groups. 

Abstract We present results from an atmospheric retrieval analysis of Gl 229B using the Brewster retrieval code. We find the best fit model to be cloud-free, consistent with the T dwarf retrieval work of Line et al.; Zalesky et al. and Gonzales et al. Fundamental parameters (mass, radius, log(L_Bol/L_Sun), log(g)) determined from our model agree within 1σto SED-derived values, except forT_effwhere our retrievedT_effis approximately 100 K cooler than the evolutionary model-based SED value. We find a retrieved mass of ${50}_{-9}^{+12}$M_Jup, however, we also find that the observables of Gl 229B can be explained by a cloud-free model with a prior on mass at the dynamical value, 70M_Jup. We are able to constrain abundances for H₂O, CO, CH₄, NH₃, Na and K and find a supersolar C/O ratio as compared to its primary, Gl 229A. We report an overall subsolar metallicity due to atmospheric oxygen depletion, but find a solar [C/H], which matches that of the primary. We find that this work contributes to a growing trend in retrieval-based studies, particularly for brown dwarfs, toward supersolar C/O ratios and discuss the implications of this result on formation mechanisms and internal physical processes, as well as model biases. 

Abstract We describe a new transit-detection algorithm designed to detect single-transit events in discontinuous Perkins INfrared Exosatellite Survey (PINES) observations of L and T dwarfs. We use this algorithm to search for transits in 131 PINES light curves and identify two transit candidates: 2MASS J18212815+1414010 (2MASS J1821+1414) and 2MASS J08350622+1953050 (2MASS J0835+1953). We disfavor 2MASS J1821+1414 as a genuine transit candidate due to the known variability properties of the source. We cannot rule out the planetary nature of 2MASS J0835+1953's candidate event and perform follow-up observations in an attempt to recover a second transit. A repeat event has yet to be observed, but these observations suggest that target variability is an unlikely cause of the candidate transit. We perform a Markov Chain Monte Carlo simulation of the light curve and estimate a planet radius ranging from 4.2 − 1.6 + 3.5 R ⊕ to 5.8 − 2.1 + 4.8 R ⊕ , depending on the host’s age. Finally, we perform an injection and recovery simulation on our light-curve sample. We inject planets into our data using measured M-dwarf planet occurrence rates and attempt to recover them using our transit-search algorithm. Our detection rates suggest that, assuming M-dwarf planet occurrence rates, we should have roughly a 1% chance of detecting a candidate that could cause the transit depth we observe for 2MASS J0835+1953. If 2MASS J0835+1953 b is confirmed, it would suggest an enhancement in the occurrence of short-period planets around L and T dwarfs in comparison to M dwarfs, which would challenge predictions from planet formation models. 

Abstract We present design considerations for the Transiting Exosatellites, Moons, and Planets in Orion (TEMPO) Survey with the Nancy Grace Roman Space Telescope. This proposed 30 days survey is designed to detect a population of transiting extrasolar satellites, moons, and planets in the Orion Nebula Cluster (ONC). The young (1–3 Myr), densely populated ONC harbors about a thousand bright brown dwarfs (BDs) and free-floating planetary-mass objects (FFPs). TEMPO offers sufficient photometric precision to monitor FFPs with M >1 M J for transiting satellites. The survey is also capable of detecting FFPs down to sub-Saturn masses via direct imaging, although follow-up confirmation will be challenging. TEMPO yield estimates include 14 (3–22) exomoons/satellites transiting FFPs and 54 (8–100) satellites transiting BDs. Of this population, approximately 50% of companions would be “super-Titans” (Titan to Earth mass). Yield estimates also include approximately 150 exoplanets transiting young Orion stars, of which >50% will orbit mid-to-late M dwarfs. TEMPO would provide the first census demographics of small exosatellites orbiting FFPs and BDs, while simultaneously offering insights into exoplanet evolution at the earliest stages. This detected exosatellite population is likely to be markedly different from the current census of exoplanets with similar masses (e.g., Earth-mass exosatellites that still possess H/He envelopes). Although our yield estimates are highly uncertain, as there are no known exoplanets or exomoons analogous to these satellites, the TEMPO survey would test the prevailing theories of exosatellite formation and evolution, which limit the certainty surrounding detection yields. 

Abstract We describe the Perkins INfrared Exosatellite Survey (PINES), a near-infrared photometric search for short-period transiting planets and moons around a sample of 393 spectroscopically confirmed L- and T-type dwarfs. PINES is performed with Boston University’s 1.8 m Perkins Telescope Observatory, located on Anderson Mesa, Arizona. We discuss the observational strategy of the survey, which was designed to optimize the number of expected transit detections, and describe custom automated observing procedures for performing PINES observations. We detail the steps of the PINES Analysis Toolkit ( PAT ), software that is used to create light curves from PINES images. We assess the impact of second-order extinction due to changing precipitable water vapor on our observations and find that the magnitude of this effect is minimized in Mauna Kea Observatories J band. We demonstrate the validity of PAT through the recovery of a transit of WASP-2 b and known variable brown dwarfs, and use it to identify a new variable L/T transition object: the T2 dwarf WISE J045746.08-020719.2. We report on the measured photometric precision of the survey and use it to estimate our transit-detection sensitivity. We find that for our median brightness targets, assuming contributions from white noise only, we are sensitive to the detection of 2.5 R ⊕ planets and larger. PINES will test whether the increase in sub-Neptune-sized planet occurrence with decreasing host mass continues into the L- and T-dwarf regime.