arXiv subjects

William M. J. Best

Publications and source records attributed to William M. J. Best.

At least 19 recordsLinked to original sources

arXiv2025

Classifying Cool Dwarfs: Comprehensive Spectral Typing of Field and Peculiar Dwarfs Using Machine Learning

Low-mass stars and brown dwarfs -- spectral types (SpTs) M0 and later -- play a significant role in studying stellar and substellar processes and demographics, reaching down to planetary-mass objects. Currently, the classification of these sources remains heavily reliant on visual inspection of spectral features, equivalent width measurements, or narrow-/wide-band spectral indices. Recent advances in machine learning (ML) methods offer automated approaches for spectral typing, which are becoming increasingly important as large spectroscopic surveys such as Gaia, SDSS, and SPHEREx generate datasets containing millions of spectra. We investigate the application of ML in spectral type classification on low-resolution (R $\sim$ 120) near-infrared spectra of M0--T9 dwarfs obtained with the SpeX instrument on the NASA Infrared Telescope Facility. We specifically aim to classify the gravity- and metallicity-dependent subclasses for late-type dwarfs. We used binned fluxes as input features and compared the efficacy of spectral type estimators built using Random Forest (RF), Support Vector Machine (SVM), and K-Nearest Neighbor (KNN) models. We tested the influence of different normalizations and analyzed the relative importance of different spectral regions for surface gravity and metallicity subclass classification. Our best-performing model (using KNN) classifies 95.5 $\pm$ 0.6% of sources to within $\pm$1 SpT, and assigns surface gravity and metallicity subclasses with 89.5 $\pm$ 0.9% accuracy. We test the dependence of signal-to-noise ratio on classification accuracy and find sources with SNR $\gtrsim$ 60 have $\gtrsim$ 95% accuracy. We also find that zy-band plays the most prominent role in the RF model, with FeH and TiO having the highest feature importance.

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arXiv2025

Strongly polarised radio pulses from a new white-dwarf-hosting long-period transient

Long-period transients (LPTs) are a new and enigmatic class of objects that produce bright pulsations in the radio, with periods far exceeding those seen in rotationally powered pulsars. The proposed progenitors for LPTs are contested, with white dwarfs or magnetars being likely candidates. Here, we present the discovery of ILT\,J163430+445010, a new LPT detected in a blind search for Stokes\,V transients in the LOFAR Two-Metre Sky Survey. Unusual for LPTs, J1634+44 shows pulses that are 100\% circularly polarised, as well as pulses that are 100\% linearly polarised, with the polarisation state changing from pulse to pulse. We detect 19 pulses in total, each with a total polarisation fraction of $\sim100\%$ and a pulse duration of at most 10\,s. The pulses show a periodicity at $841.24808\pm0.00015$\,s, implying a low duty cycle of $0.012$. J1634+44 has a marginally detected counterpart in the ultraviolet GALEX MIS survey and the ultraviolet/optical UNIONS survey, suggesting that it contains a white dwarf with an effective temperature between 15000\,K and 33000\,K. We do not detect J1634+44 with a deep $J$-band exposure with UKIRT at a $3σ$ AB magnitude limit of 24.7, ruling out a main-sequence star or ultracool dwarf with a spectral type earlier than M7. The pulses from J1634+44 follow a particular pattern, with two pulses being produced every five periods after a waiting time of two or three periods. This pattern could be a result of spin-orbit coupling in a binary system with a 5:2 or 5:3 resonance, where a companion induces beamed radio emission on the white dwarf. The companion is most likely an ultracool dwarf or another white dwarf, making J1634+44 unique among the currently known sample of LPTs.

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arXiv2024

Disequilibrium Chemistry, Diabatic Thermal Structure, and Clouds in the Atmosphere of COCONUTS-2b

Located 10.888 pc from Earth, COCONUTS-2b is a planetary-mass companion to a young (150-800 Myr) M3 star, with a wide orbital separation (6471 au) and a low companion-to-host mass ratio ($0.021\pm0.005$). We have studied the atmospheric properties of COCONUTS-2b using newly acquired 1.0-2.5 $μ$m spectroscopy from Gemini/Flamingos-2. The spectral type of COCONUTS-2b is refined to T$9.5 \pm 0.5$ based on comparisons with T/Y dwarf spectral templates. We have conducted an extensive forward-modeling analysis, comparing the near-infrared spectrum and mid-infrared broadband photometry with sixteen state-of-the-art atmospheric model grids developed for brown dwarfs and self-luminous exoplanets near the T/Y transition. The PH$_{3}$-free ATMO2020++, ATMO2020++, and Exo-REM models best match the specific observations of COCONUTS-2b, regardless of variations in the input spectrophotometry. This analysis suggests the presence of disequilibrium chemistry, along with a diabatic thermal structure and/or clouds, in the atmosphere of COCONUTS-2b. All models predict fainter $Y$-band fluxes than observed, highlighting uncertainties in the alkali chemistry models and opacities. We determine a bolometric luminosity of $\log{(L_{\rm bol}/L_{\odot})}=-6.18$ dex, with a 0.5 dex-wide range of $[-6.43,-5.93]$ dex that accounts for various assumptions of models. Using thermal evolution models, we derive an effective temperature of $T_{\rm eff}=483^{+44}_{-53}$ K, a surface gravity of $\log{(g)}=4.19^{+0.18}_{-0.13}$ dex, a radius of $R=1.11^{+0.03}_{-0.04}$ R$_{\rm Jup}$, and a mass of $M=8 \pm 2$ M$_{\rm Jup}$. Various atmospheric model grids consistently indicate that COCONUTS-2b's atmosphere has sub- or near-solar metallicity and C/O. These findings provide valuable insights into COCONUTS-2b's formation history and the potential outward migration to its current wide orbit.

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arXiv2024

The TEMPO Survey II: Science Cases Leveraged from a Proposed 30-Day Time Domain Survey of the Orion Nebula with the Nancy Grace Roman Space Telescope

The TEMPO (Transiting Exosatellites, Moons, and Planets in Orion) Survey is a proposed 30-day observational campaign using the Nancy Grace Roman Space Telescope. By providing deep, high-resolution, short-cadence infrared photometry of a dynamic star-forming region, TEMPO will investigate the demographics of exosatellites orbiting free-floating planets and brown dwarfs -- a largely unexplored discovery space. Here, we present the simulated detection yields of three populations: extrasolar moon analogs orbiting free-floating planets, exosatellites orbiting brown dwarfs, and exoplanets orbiting young stars. Additionally, we outline a comprehensive range of anticipated scientific outcomes accompanying such a survey. These science drivers include: obtaining observational constraints to test prevailing theories of moon, planet, and star formation; directly detecting widely separated exoplanets orbiting young stars; investigating the variability of young stars and brown dwarfs; constraining the low-mass end of the stellar initial mass function; constructing the distribution of dust in the Orion Nebula and mapping evolution in the near-infrared extinction law; mapping emission features that trace the shocked gas in the region; constructing a dynamical map of Orion members using proper motions; and searching for extragalactic sources and transients via deep extragalactic observations reaching a limiting magnitude of $m_{AB}=29.7$\,mag (F146 filter).

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arXiv2024

A Volume-Limited Sample of Ultracool Dwarfs. II. The Substellar Age and Mass Functions in the Solar Neighborhood

We present the most precise constraints to date for the mass and age distributions of single ultracool dwarfs in the solar neighborhood, based on an updated volume-limited sample of 504 L, T, and Y dwarfs within 25 pc. We develop a Monte Carlo approach using the $\langle V/V_{\rm max}\rangle$ statistic to correct for incompleteness and obtain a space density of $(1.83_{-0.15}^{+0.16})\times10^{-2}$ pc$^{-3}$ for spectral types L0-Y2. We calculate bolometric luminosities for our sample, using an updated "super-magnitude" method for the faintest objects. We use our resulting luminosity function and a likelihood-based population synthesis approach to simultaneously constrain the mass and age distributions. We employ the fraction of young L0-L7 dwarfs as a novel input for this analysis that is crucial for constraining the age distribution. For a power-law mass function $\frac{dN}{dM} \propto M^{-α}$ we find $α=0.58_{-0.20}^{+0.16}$, indicating an increase in numbers toward lower masses, consistent with measurements in nearby star-forming regions. For an exponential age distribution $b(t) \propto e^{-βt}$ we find $β=-0.44\pm0.14$, i.e., a population with fewer old objects than often assumed, which may reflect dynamical heating of the Galactic plane as much as the historical brown dwarf birthrate. We compare our analysis to Kirkpatrick et al. (2021), who used a similar volume-limited sample. Although our mass function measurements are numerically consistent, their assumption of a flat age distribution is disfavored by our analysis, and we identify several important methodological differences between our two studies. Our calculation of the age distribution of solar neighborhood brown dwarfs is the first based on a volume-limited sample.

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arXiv2023

Uniform Forward-Modeling Analysis of Ultracool Dwarfs. III. Late-M and L Dwarfs in Young Moving Groups, the Pleiades, and the Hyades

We present a uniform forward-modeling analysis of 90 late-M and L dwarfs in nearby young (~$10-200$ Myr) moving groups, the Pleiades, and the Hyades using low-resolution ($R\approx150$) near-infrared ($0.9-2.4$ $\mathrm{μm}$) spectra and the BT-Settl model atmospheres. We derive the objects' effective temperatures, surface gravities, radii, and masses by comparing our spectra to the models using a Bayesian framework with nested sampling and calculate the same parameters using evolutionary models. Assuming the evolutionary-based parameters are more robust, our spectroscopically inferred parameters from BT-Settl exhibit two types of systematic behavior for objects near the M-L spectral type boundary. Several are clustered around $T_\mathrm{eff} \approx 1800$ K and $\log g\approx5.5$ dex, implying impossibly large masses ($150-1400$ $M_\mathrm{Jup}$), while others are clustered around $T_\mathrm{eff}\gtrsim3000$ K and $\log g\lesssim3.0$ dex, implying non-physical low masses and unreasonably young ages. We find the fitted BT-Settl model spectra tend to overpredict the peak $J$ and $H$-band flux for objects located near the M-L boundary, suggesting the dust content included in the model atmospheres is insufficient to match the observations. By adding an interstellar medium-like reddening law to the BT-Settl model spectra, we find the fits between models and observed spectra are greatly improved, with the largest reddening coefficients occurring at the M-L transition. This work delivers a systematic examination of the BT-Settl model atmospheres and constitutes the largest spectral analysis of benchmark late-M and L-type brown dwarfs to date.

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arXiv2023

On the Masses, Age, and Architecture of the VHS J1256-1257AB b System

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.

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arXiv2022

The TEMPO Survey I: Predicting Yields of the Transiting Exosatellites, Moons, and Planets from a 30-day Survey of Orion with the Nancy Grace Roman Space Telescope

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-day 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 ${\rm M}\geq1{\rm M}_{\rm J}$ 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 and approximately ten will be proto-habitable zone, terrestrial ($0.1{\rm M}_{\oplus} - 5{\rm M}_{\oplus}$) exoplanets. 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.

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arXiv2022

ALMA Discovery of a Disk around the Planetary-mass Companion SR 12 c

We report an Atacama Large Millimeter/submillimeter Array 0.88 mm (Band 7) continuum detection of the accretion disk around SR 12 c, an $\sim$11 $M_{\rm Jup}$ planetary-mass companion (PMC) orbiting its host binary at 980 au. This is the first submillimeter detection of a circumplanetary disk around a wide PMC. The disk has a flux density of $127 \pm14~μ$Jy and is not resolved by the $\sim$0.1" beam, so the dust disk radius is likely less than 5 au and can be much smaller if the dust continuum is optically thick. If, however, the dust emission is optically thin, then the SR 12 c disk has a comparable dust mass to the circumplanetary disk around PDS 70 c but is about five times lower than that of the $\sim$12 $M_{\rm Jup}$ free-floating OTS 44. This suggests that disks around bound and unbound planetary-mass objects can span a wide range of masses. The gas mass estimated with an accretion rate of $10^{-11}~M_\odot$ yr$^{-1}$ implies a gas-to-dust ratio higher than 100. If cloud absorption is not significant, a nondetection of ${}^{12}$CO(3-2) implies a compact gas disk around SR 12 c. Future sensitive observations may detect more PMC disks at 0.88 mm flux densities of $\lesssim$100 $μ$Jy.

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arXiv2022

Using Old and New Approaches: Determining Physical Properties of Brown Dwarfs with Empirical Relations and Machine Learning Models

We investigate applications of machine learning models to directly infer physical properties of brown dwarfs from their photometry and spectra using $\textit{The Cannon}$. We demonstrate that absolute magnitudes, spectral types, and spectral indices can be determined from low-resolution SpeX prism spectra of L and T dwarfs without trigonometric parallax measurements and with precisions competitive with commonly used methods. For T dwarfs with sufficiently precise spectra and photometry, bolometric luminosities and effective temperatures can be determined at precisions comparable to methods that use polynomial relations as a function of absolute magnitudes. We also provide new and updated polynomial relations for absolute magnitudes as a function of spectral types L0-T8 in 14 bands spanning Pan-STARRS $r_{P1}$ to AllWISE $\textit{W3}$, using a volume-limited sample of 256 brown dwarfs defined entirely by parallaxes. These include the first relations for brown dwarfs using Pan-STARRS1 photometry and the first for several infrared bands using a volume-limited sample. We find that our novel method with $\textit{The Cannon}$ can infer absolute magnitudes with equal or smaller uncertainties than the polynomial relations that depend on trigonometric parallax measurements.

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arXiv2021

Improved Dynamical Masses for Six Brown Dwarf Companions Using Hipparcos and Gaia EDR3

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 $<$1\% mass measurement of $71.4 \pm 0.6\,M_{\rm Jup}$ for the first T dwarf Gl~229~B and a 1.2\% mass measurement of its host star ($0.579 \pm 0.007\,M_{\odot}$) that agrees with the high-mass-end of the M dwarf mass-luminosity relation. We perform a homogeneous analysis of the host stars' ages and use them, along with the companions' measured masses and luminosities, to test substellar evolutionary models. Gl~229~B is the most discrepant, as models predict that an object this massive cannot cool to such a low luminosity within a Hubble time, implying that it may be an unresolved binary. The other companions are generally consistent with models, except for HD~13724~B that has a host-star activity age 3.8$σ$ older than its substellar cooling age. Examining our results in context with other mass-age-luminosity benchmarks, we find no trend with spectral type but instead note that younger or lower-mass brown dwarfs are over-luminous compared to models, while older or higher-mass brown dwarfs are under-luminous. The presented mass measurements for some companions are so precise that the stellar host ages, not the masses, limit the analysis.

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arXiv2021

The Second Discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) Program: A Cold Wide-Orbit Exoplanet around a Young Field M Dwarf at 10.9 pc

We present the identification of the COCONUTS-2 system, composed of the M3 dwarf L 34-26 and the T9 dwarf WISEPA J075108.79$-$763449.6. Given their common proper motions and parallaxes, these two field objects constitute a physically bound pair with a projected separation of 594$"$ (6471 au). The primary star COCONUTS-2A has strong stellar activity (H$α$, X-ray, and UV emission) and is rapidly rotating ($P_{\rm rot} = 2.83$ days), from which we estimate an age of 150-800 Myr. Comparing equatorial rotational velocity derived from the TESS light curve to spectroscopic $v\sin{i}$, we find COCONUTS-2A has a nearly edge-on inclination. The wide exoplanet COCONUTS-2b has an effective temperature of $T_{\rm eff}=434 \pm 9$ K, a surface gravity of $\log{g} = 4.11^{+0.11}_{-0.18}$ dex, and a mass of $M=6.3^{+1.5}_{-1.9}$ $M_{\rm Jup}$ based on hot-start evolutionary models, leading to a $0.016^{+0.004}_{-0.005}$ mass ratio for the COCONUTS-2 system. COCONUTS-2b is the second coldest (after WD 0806$-$661B) and the second widest (after TYC 9486-927-1 b) exoplanet imaged to date. Comparison of COCONUTS-2b's infrared photometry with ultracool model atmospheres suggests the presence of both condensate clouds and non-equilibrium chemistry in its photosphere. Similar to 51 Eri b, COCONUTS-2b has a sufficiently low luminosity ($\log{(L_{\rm bol}/L_{\odot})} = -6.384 \pm 0.028$ dex) to be consistent with the cold-start process that may form gas-giant (exo)planets, though its large separation means such formation would not have occurred in situ. Finally, at a distance of 10.9 pc, COCONUTS-2b is the nearest imaged exoplanet to Earth known to date.

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arXiv2021

Uniform Forward-Modeling Analysis of Ultracool Dwarfs. I. Methodology and Benchmarking

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 ($1.0-2.5$ $μ$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 ($χ^2$-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.

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arXiv2021

Uniform Forward-Modeling Analysis of Ultracool Dwarfs. II. Atmospheric Properties of 55 Late-T Dwarfs

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 $μ$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.

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arXiv2021

The Hawaii Infrared Parallax Program. V. New T-Dwarf Members and Candidate Members of Nearby Young Moving Groups

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 $Σ$ 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.

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arXiv2020

A Volume-Limited Sample of Ultracool Dwarfs. I. Construction, Space Density, and a Gap in the L/T Transition

We present a new volume-limited sample of L0-T8 dwarfs out to 25 pc defined entirely by parallaxes, using our recent measurements from UKIRT/WFCAM along with Gaia DR2 and literature parallaxes. With 369 members, our sample is the largest parallax-defined volume-limited sample of L and T dwarfs to date, yielding the most precise space densities for such objects. We find the local L0-T8 dwarf population includes $5.5\%\pm1.3\%$ young objects ($\lesssim$200 Myr) and $2.6\%\pm1.6\%$ subdwarfs, as expected from recent studies favoring representative ages $\lesssim$4 Gyr for the ultracool field population. This is also the first volume-limited sample to comprehensively map the transition from L to T dwarfs (spectral types $\approx$L8-T4). After removing binaries, we identify a previously unrecognized, statistically significant (>4.4$σ$) gap $\approx$0.5 mag wide in $(J-K)_{\rm MKO}$ colors in the L/T transition, i.e., a lack of such objects in our volume-limited sample, implying a rapid phase of atmospheric evolution. In contrast, the most successful models of the L/T transition to date $-$ the "hybrid" models of Saumon & Marley (2008) $-$ predict a pile-up of objects at the same colors where we find a deficit, demonstrating the challenge of modeling the atmospheres of cooling brown dwarfs. Our sample illustrates the insights to come from even larger parallax-selected samples from the upcoming Legacy Survey of Space and Time (LSST) by the Vera Rubin Obsevatory.

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arXiv2020

Direct radio discovery of a cold brown dwarf

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.

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arXiv2020

The Hawaii Infrared Parallax Program. IV. A Comprehensive Parallax Survey of L0-T8 dwarfs with UKIRT

We present parallaxes, proper motions, and $J$-band photometry for 348 L and T dwarfs measured using the wide-field near-infrared camera WFCAM on the United Kingdom Infrared Telescope. This is the largest single batch of infrared parallaxes for brown dwarfs to date. Our parallaxes have a median uncertainty of 3.5 mas, similar to most previous ground-based infrared parallax surveys. Our target list was designed to complete a volume-limited parallax sample of L0-T8 dwarfs out to 25 pc spanning declinations $-30^\circ$ to $+60^\circ$ (68% of the sky). We report the first parallaxes for 165 objects, and we improve on previous measurements for another 53 objects. Our targets include 104 objects (mostly early-L dwarfs) having $Gaia$ DR2 parallax measurements, with which our parallaxes are consistent. We include an extensive comparison of previous literature parallaxes for L and T dwarfs with both our results and $Gaia$ DR2 measurements, identifying systematic offsets for some previous surveys. Our parallaxes confirm that 14 objects previously identified as wide common proper motion companions to main-sequence stars have distances consistent with companionship. We also report new $J_\mathrm{MKO}$ photometry for our targets, including the first measurements for 193 of our targets and improvements over previously published $J_\mathrm{MKO}$ photometry for another 60 targets. Altogether, our parallaxes will enable the first population studies using a volume-limited sample spanning spectral types L0-T8 defined entirely by parallaxes.

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