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We report the discovery of a high-velocity, very low-mass star or brown dwarf whose kinematics suggest it is unbound to the Milky Way. CWISE J124909.08+362116.0 was identified by citizen scientists in the Backyard Worlds: Planet 9 program as a high-proper-motion ( μ = 0.″9 yr ^−1 ) faint red source. Moderate-resolution spectroscopy with Keck/NIRES reveals it to be a metal-poor early L subdwarf with a large radial velocity (−103 ± 10 km s ^−1 ), and its estimated distance of 125 ± 8 pc yields a speed of 456 ± 27 km s ^−1 in the Galactic rest frame, near the local escape velocity for the Milky Way. We explore several potential scenarios for the origin of this source, including ejection from the Galactic center ≳3 Gyr in the past, survival as the mass donor companion to an exploded white dwarf, acceleration through a three-body interaction with a black hole binary in a globular cluster, and accretion from a Milky Way satellite system. CWISE J1249+3621 is the first hypervelocity very low-mass star or brown dwarf to be found and the nearest of all such systems. It may represent a broader population of very high-velocity, low-mass objects that have undergone extreme accelerations.

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

After decades of brown dwarf discovery and follow-up, we can now infer the functional form of the mass distribution within 20 pc, which serves as a constraint on star formation theory at the lowest masses. Unlike objects on the main sequence that have a clear luminosity-to-mass correlation, brown dwarfs lack a correlation between an observable parameter (luminosity, spectral type, or color) and mass. A measurement of the brown dwarf mass function must therefore be procured through proxy measurements and theoretical models. We utilize various assumed forms of the mass function, together with a variety of birthrate functions, low-mass cutoffs, and theoretical evolutionary models, to build predicted forms of the effective temperature distribution. We then determine the best fit of the observed effective temperature distribution to these predictions, which in turn reveals the most likely mass function. We find that a simple power law ( ${dN}/{dM}\propto {M}^{-\alpha }$ ) with α ≈ 0.5 is optimal. Additionally, we conclude that the low-mass cutoff for star formation is ≲0.005 M _⊙ . We corroborate the findings of Burgasser, which state that the birthrate has a far lesser impact than the mass function on the form of the temperature distribution, but we note that our alternate birthrates tend to favor slightly smaller values of α than the constant birthrate. Our code for simulating these distributions is publicly available. As another use case for this code, we present findings on the width and location of the subdwarf temperature gap by simulating distributions of very old (8–10 Gyr) brown dwarfs.

We have used the UKIRT Hemisphere Survey combined with the UKIDSS Galactic Cluster Survey, the UKIDSS Galactic Plane Survey, and the CatWISE2020 catalog to search for new substellar members of the nearest open cluster to the Sun, the Hyades. Eight new substellar Hyades candidate members were identified and observed with the Gemini/GNIRS near-infrared spectrograph. All eight objects are confirmed as brown dwarfs with spectral types ranging from L6 to T5, with two objects showing signs of spectral binarity and/or variability. A kinematic analysis demonstrates that all eight new discoveries likely belong to the Hyades cluster, with future radial velocity and parallax measurements needed to confirm their membership. CWISE J042356.23+130414.3, with a spectral type of T5, would be the coldest ( T _eff ≈ 1100 K) and lowest-mass ( M ≈ 30 M _Jup ) free-floating member of the Hyades yet discovered. We further find that high-probability substellar Hyades members from this work and previous studies have redder near-infrared colors than field-age brown dwarfs, potentially due to lower surface gravities and supersolar metallicities.

We present infrared spectral energy distributions of 23 late-type T and Y dwarfs obtained with the James Webb Space Telescope. The spectral energy distributions consist of NIRSpec PRISM and MIRI low-resolution spectrometer spectra covering the ∼1–12 μ m wavelength range at λ /Δ λ ≈ 100 and broadband photometry at 15, 18, and 21 μ m. The spectra exhibit absorption features common to these objects including H _2 O, CH _4 , CO, CO _2 , and NH _3 . Interestingly, while the spectral morphology changes relatively smoothly with spectral type at λ < 3 μ m and λ > 8 μ m, it shows no clear trend in the 5 μ m region where a large fraction of the flux emerges. The broad-wavelength coverage of the data enables us to compute the first accurate measurements of the bolometric fluxes of cool brown dwarfs. Combining these bolometric fluxes with parallaxes from Spitzer and Hubble Space Telescope, we also obtain the first accurate bolometric luminosities of these cool dwarfs. We then used the Sonora Bobcat solar-metallicity evolutionary models to estimate the radii of the dwarfs, which results in effective temperature estimates ranging from ∼1000 to 350 K with a median uncertainty of ± 20 K, which is nearly an order of magnitude improvement over previous work. We also discuss how various portions of the spectra either do or do not exhibit a clear sequence when ordered by their effective temperatures.

The sensitivity and spectral coverage of JWST are enabling us to test our assumptions of ultracool dwarf atmospheric chemistry, especially with regards to the abundances of phosphine (PH _3 ) and carbon dioxide (CO _2 ). In this paper, we use Near Infrared Spectrograph PRISM spectra (∼0.8−5.5 μ m, R ∼ 100) of four late T and Y dwarfs to show that standard substellar atmosphere models have difficulty replicating the 4.1−4.4 μ m wavelength range, as they predict an overabundance of phosphine and an underabundance of carbon dioxide. To help quantify this discrepancy, we generate a grid of models using PICASO, based on the Elf Owl chemical and temperature profiles, where we include the abundances of these two molecules as parameters. The fits to these PICASO models show a consistent preference for orders-of-magnitude higher CO _2 abundances and a reduction in PH _3 abundance as compared to the nominal models. This tendency means that the claimed phosphine detection in UNCOVER−BD−3 could instead be explained by a CO _2 abundance in excess of standard atmospheric model predictions; however, the signal-to-noise ratio of the spectrum is not high enough to discriminate between these cases. We discuss atmospheric mechanisms that could explain the observed underabundance of PH _3 and overabundance of CO _2 , including a vertical eddy diffusion coefficient ( K _zz ) that varies with altitude, incorrect chemical pathways, or elements condensing out in forms such as NH _4 H _2 PO _4 . However, our favored explanation for the required CO _2 enhancement is that the quench approximation does not accurately predict the CO _2 abundance, as CO _2 remains in chemical equilibrium with CO after CO quenches.

We present an atmospheric retrieval analysis of the Y0 brown dwarf WISE J035934.06−540154.6 using the low-resolution 0.96–12 μ m James Webb Space Telescope (JWST) spectrum presented in Beiler et al. We obtain volume number mixing ratios of the major gas-phase absorbers (H _2 O, CH _4 , CO, CO _2 , PH _3 , and H _2 S) that are three to five times more precise than previous work that used Hubble Space Telescope (HST) spectra. We also find an order-of-magnitude improvement in the precision of the retrieved thermal profile, a direct result of the broad wavelength coverage of the JWST data. We used the retrieved thermal profile and surface gravity to generate a grid of chemical forward models with varying metallicity, (C/O) _atm , and strengths of vertical mixing as encapsulated by the eddy diffusion coefficient K _zz . Comparison of the retrieved abundances with this grid of models suggests that the deep atmosphere of WISE 0359−54 shows signs of vigorous vertical mixing with K _zz = 10 ^9 [cm ^2 s ^−1 ]. To test the sensitivity of these results to our five-knot spline thermal profile model, we performed a second retrieval using the Madhusudhan & Seager thermal profile model. While the results of the two retrievals generally agree well, we do find differences between the retrieved values of mass and volume number mixing ratio of H _2 S with fractional differences of the median values of −0.64 and −0.10, respectively. In addition, the five-knot thermal profile is consistently warmer at pressure between 1 and 70 bar. Nevertheless, our results underscore the power that the broad-wavelength infrared spectra obtainable with the JWST have to characterize the atmospheres of cool brown dwarfs.

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 − W 2 = 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 CH _4 absorption features, indicating an exceptionally low effective temperature for its spectral type. Based on proper-motion measurements from CatWISE2020 and a photometric distance derived from its K _s -band magnitude, we find that VHS J1831−5513 is a likely (∼85% probability) kinematic member of the β Pictoris 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.

We present the discovery of 13 new widely separated T dwarf companions to M dwarf primaries, identified using Wide-field Infrared Survey Explorer/NEOWISE data by the CatWISE and Backyard Worlds: Planet 9 projects (hereafter BYW). This sample represents an ∼60% increase in the number of known M + T systems, and allows us to probe the most extreme products of binary/planetary system formation, a discovery space made available by the CatWISE2020 catalog and the BYW effort. Highlights among the sample are WISEP J075108.79-763449.6, a previously known T9 thought to be old due to its spectral energy distribution, which was found by Zhang et al. (2021b) to be part of a common proper motion pair with L34-26 A, a well-studied young M3 V star within 10 pc of the Sun; CWISE J054129.32-745021.5 B and 2MASS J05581644-4501559 B, two T8 dwarfs possibly associated with the very fast-rotating M4 V stars CWISE J054129.32745021.5 A and 2MASS J05581644-4501559 A; and UCAC3 52-1038 B, which is among the widest late-T companions to main-sequence stars, with a projected separation of ∼7100 au. The new benchmarks presented here are prime JWST targets, and can help us place strong constraints on the formation and evolution theory of substellar objects as well as on atmospheric models for these cold exoplanet analogs.

We report the identification of 89 new systems containing ultracool dwarf companions to main-sequence stars and white dwarfs, using the citizen science project Backyard Worlds: Planet 9 and cross-reference between Gaia and CatWISE2020. 32 of these companions and 33 host stars were followed up with spectroscopic observations, with companion spectral types ranging from M7–T9 and host spectral types ranging from G2–M9. These systems exhibit diverse characteristics, from young to old ages, blue to very red spectral morphologies, potential membership to known young moving groups, and evidence of spectral binarity in nine companions. 20 of the host stars in our sample show evidence for higher-order multiplicity, with an additional 11 host stars being resolved binaries themselves. We compare this sample’s characteristics with those of the known stellar binary and exoplanet populations, and find our sample begins to fill in the gap between directly imaged exoplanets and stellar binaries on mass ratio–binding energy plots. With this study, we increase the population of ultracool dwarf companions to FGK stars by ∼42%, and more than triple the known population of ultracool dwarf companions with separations larger than 1000 au, providing excellent targets for future atmospheric retrievals.

A complete accounting of nearby objects—from the highest-mass white dwarf progenitors down to low-mass brown dwarfs—is now possible, thanks to an almost complete set of trigonometric parallax determinations from Gaia, ground-based surveys, and Spitzer follow-up. We create a census of objects within a Sun-centered sphere of 20 pc radius and check published literature to decompose each binary or higher-order system into its separate components. The result is a volume-limited census of ∼3600 individual star formation products useful in measuring the initial mass function across the stellar (

We present the first JWST spectral energy distribution of a Y dwarf. This spectral energy distribution of the Y0 dwarf WISE J035934.06−540154.6 consists of low-resolution ( λ /Δ λ ∼100) spectroscopy from 1–12 μ m and three photometric points at 15, 18, and 21 μ m. The spectrum exhibits numerous fundamental, overtone, and combination rotational–vibrational bands of H _2 O, CH _4 , CO, CO _2 , and NH _3 , including the previously unidentified ν _3 band of NH _3 at 3 μ m. Using a Rayleigh–Jeans tail to account for the flux emerging at wavelengths greater than 21 μ m, we measure a bolometric luminosity of 1.523 ± 0.090 × 10 ^20 W. We determine a semiempirical effective temperature estimate of ${467}_{-18}^{+16}$ K using the bolometric luminosity and evolutionary models to estimate a radius. Finally, we compare the spectrum and photometry to a grid of atmospheric models and find reasonably good agreement with a model having T _eff = 450 K, log g = 3.25 [cm s ^−2 ], and [M/H] = −0.3. However, the low surface gravity implies an extremely low mass of 1 M _Jup and a very young age of 20 Myr, the latter of which is inconsistent with simulations of volume-limited samples of cool brown dwarfs.

We present the discovery of CWISE J050626.96+073842.4 (CWISE J0506+0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 citizen science project. Photometry from UKIRT and CatWISE give a ( J − K ) _MKO color of 2.97 ± 0.03 mag and a J _MKO − W2 color of 4.93 ± 0.02 mag, making CWISE J0506+0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506+0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity, late-type L/T transition dwarf. The spectrum of CWISE J0506+0738 shows possible signatures of CH _4 absorption in its atmosphere, suggesting a colder effective temperature than other known, young, red L dwarfs. We assign a preliminary spectral type for this source of L8 γ –T0 γ . We tentatively find that CWISE J0506+0738 is variable at 3–5 μ m based on multiepoch WISE photometry. Proper motions derived from follow-up UKIRT observations combined with a radial velocity from our Keck/NIRES spectrum and a photometric distance estimate indicate a strong membership probability in the β Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object.

The extensive diversity of the tribe Orobancheae, the most species-rich lineage of holoparasitic Orobanchaceae, is concentrated in the Caucasus and Mediterranean regions of the Old World. This extant diversity has inspired hypotheses that these regions are also centres of origin of its key lineages, however the ability to test hypotheses has been limited by a lack of sampling and phylogenetic information about the species, especially in the Caucasus region. First, we assessed the phylogenetic relationships of several poorly known, problematic, or newly described species and host-races of four genera of Orobancheae occurring in the Caucasus region–Cistanche, Phelypaea, Phelipanche and Orobanche–using nuclear ribosomal (ITS) and plastid (trnL–trnF) sequence data. Then we applied a probablistic dispersal-extinction-cladogenesis model of historical biogeography across a more inclusive clade of holoparasites, to explicitly test hypotheses of Orobancheae diversification and historical biogeography shifts. In sum, we sampled 548 sequences (including 196 newly generated) from 13 genera, 140 species, and 175 taxa across 44 countries. We find that the Western Asia (particularly the Caucasus) and the Mediterranean are the centre of origin for large clades of holoparasitic Orobancheae within the last 6 million years. In the Caucasus, the centres of diversity are composed both of long-branch taxa and shallow, recently diversified clades, while Orobancheae diversity in the Mediterranean appears to represent mainly recent diversification.

We present three new brown dwarf spectral-binary candidates: CWISE J072708.09−360729.2, CWISE J103604.84−514424.4, and CWISE J134446.62−732053.9, discovered by citizen scientists through the Backyard Worlds: Planet 9 project. Follow-up near-infrared spectroscopy shows that each of these objects is poorly fit by a single near-infrared standard. We constructed binary templates and found significantly better fits, with component types of L7+T4 for CWISE J072708.09−360729.2, L7+T4 for CWISE J103604.84−514424.4, and L7+T7 for CWISE J134446.62−732053.9. However, further investigation of available spectroscopic indices for evidence of binarity and large amplitude variability suggests that CWISE J072708.09−360729.2 may instead be a strong variability candidate. Our analysis offers tentative evidence and characterization of these peculiar brown dwarf sources, emphasizing their value as promising targets for future high-resolution imaging or photometric variability studies.

Low-mass stars (≤1 M _⊙ ) are some of the best candidates for hosting planets with detectable life because of these stars’ long lifetimes and relative ratios of planet to star mass and radius. An important aspect of these stars to consider is the amount of ultraviolet (UV) and X-ray radiation incident on planets in the habitable zones due to the ability of UV and X-ray radiation to alter the chemistry and evolution of planetary atmospheres. In this work, we build on the results of the HAZMAT I and HAZMAT III M-star studies to determine the intrinsic UV and X-ray flux evolution with age for M stars using Gaia parallactic distances. We then compare these results to the intrinsic fluxes of K stars adapted from HAZMAT V. We find that although the intrinsic M-star UV flux is 10–100 times lower than that of K stars, the UV fluxes in their respective habitable zone are similar. However, the habitable zone X-ray flux evolutions are slightly more distinguishable with a factor of 3–15 times larger X-ray flux for late M stars than for K stars. These results suggest that there may not be a K-dwarf advantage compared to M stars in the UV, but one may still exist in the X-ray.

We present the positions, proper motions, and near-infrared photometry for 966 known objects with spectral types later than M that were observed as part of the the UKIRT Hemisphere Survey (UHS). We augment the photometry and astrometry from UHS with information from Gaia DR3, Pan-STARRS DR2, and CatWISE 2020 to produce a database of homogeneous photometry and astrometry for this sample. The multi-epoch survey strategy of UHS allows us to determine the proper motions for most sources, with a median proper motion uncertainty of ∼3.6 mas yr ^−1 . Our UHS proper motion measurements are generally in good agreement with those from Gaia DR3, Pan-STARRS, and CatWISE 2020, with UHS proper motions typically more precise than those from CatWISE 2020 and Pan-STARRS but not Gaia DR3. We critically analyze the publicly available spectra for 406 members of this sample and provide updated near-infrared spectral types for ∼100 objects. We determine typical colors as a function of spectral type and provide absolute magnitude versus spectral type relations for UHS J - and K -band photometry. Using newly determined proper motions, we highlight several objects of interest, such as objects with large tangential velocities, widely separated co-moving companions, and potential members of young nearby associations.

Mapping out the populations of thick disk and halo brown dwarfs is important for understanding the metallicity dependence of low-temperature atmospheres and the substellar mass function. Recently, a new population of cold and metal-poor brown dwarfs has been discovered, with T _eff ≲ 1400 K and metallicity ≲−1 dex. This population includes what may be the first known “extreme T-type subdwarfs” and possibly the first Y-type subdwarf, WISEA J153429.75−104303.3. We have conducted a Gemini YJHK / Ks photometric follow-up campaign targeting potentially metal-poor T and Y dwarfs, utilizing the GNIRS and Flamingos-2 instruments. We present 14 near-infrared photometric detections of eight unique targets: six T subdwarf candidates, one moderately metal-poor Y dwarf candidate, and one Y subdwarf candidate. We have obtained the first-ever ground-based detection of the highly anomalous object WISEA J153429.75−104303.3. The F110W − J color of WISEA J153429.75−104303.3 is significantly bluer than that of other late T and Y dwarfs, indicating that WISEA J153429.75−104303.3 has an unusual spectrum in the 0.9–1.4 μ m wavelength range which encompasses the J -band peak. Our J -band detection of WISEA J153429.75−104303.3 and corresponding model comparisons suggest a subsolar metallicity and temperature of 400–550 K for this object. JWST spectroscopic follow-up at near-infrared and mid-infrared wavelengths would allow us to better understand the spectral peculiarities of WISEA J153429.75−104303.3, assess its physical properties, and conclusively determine whether or not it is the first Y-type subdwarf.

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

The HR 2562 system is a rare case where a brown dwarf companion resides in a cleared inner hole of a debris disk, offering invaluable opportunities to study the dynamical interaction between a substellar companion and a dusty disk. We present the first ALMA observation of the system as well as the continued Gemini Planet Imager monitoring of the companion’s orbit with six new epochs from 2016 to 2018. We update the orbital fit, and in combination with absolute astrometry from GAIA, place a 3 σ upper limit of 18.5 M _J on the companion’s mass. To interpret the ALMA observations, we used radiative transfer modeling to determine the disk properties. We find that the disk is well resolved and nearly edge-on. While the misalignment angle between the disk and the orbit is weakly constrained, due to the short orbital arc available, the data strongly support a (near) coplanar geometry for the system. Furthermore, we find that the models that describe the ALMA data best have inner radii that are close to the companion’s semimajor axis. Including a posteriori knowledge of the system’s SED further narrows the constraints on the disk’s inner radius and places it at a location that is in reasonable agreement with (possibly interior to) predictions from existing dynamical models of disk truncation by an interior substellar companion. HR 2562 has the potential over the next few years to become a new test bed for dynamical interaction between a debris disk and a substellar companion.

Variability in the far-ultraviolet (FUV) emission produced by stellar activity affects photochemistry and heating in orbiting planetary atmospheres. We present a comprehensive analysis of the FUV variability of GJ 436, a field-age M2.5V star ( P _rot ≈ 44 days) that is orbited by a warm Neptune-sized planet ( M ≈ 25 M _⊕ , R ≈ 4.1 M _⊕ , P _orb ≈ 2.6 days). Observations at three epochs from 2012 to 2018 span nearly a full activity cycle, sample two rotations of the star and two orbital periods of the planet, and reveal a multitude of brief flares. From 2012 to 2018, the star’s 7.75 ± 0.10 yr activity cycle produced the largest observed variations, 38% ± 3% in the summed flux of the major FUV emission lines. In 2018, the variability due to rotation was 8% ± 2%. An additional 11% ± 1% scatter at a cadence of 10 minutes, which is treated as white noise in the fits, likely has both instrumental and astrophysical origins. Flares increased time-averaged emission by 15% over the 0.88 days of cumulative exposure, peaking as high as 25× quiescence. We interpret these flare values as lower limits given that flares too weak or too infrequent to have been observed likely exist. GJ 436’s flare frequency distribution at FUV wavelengths is unusual compared to other field-age M dwarfs, exhibiting a statistically significant dearth of high-energy (>4 × 10 ^28 erg) events, which we hypothesize to be the result of a magnetic star–planet interaction (SPI) triggering premature flares. If an SPI is present, GJ 436 b’s magnetic field strength must be ≲100 G to explain the statistically insignificant increase in the orbit-phased FUV emission.

Recent phylogenetic studies support a monophyletic clade of New World broomrapes (Orobanche sects. Gymnocaulis and Nothaphyllon) sister to the Old World genus Phelipanche. I place the New World taxa in the genus Aphyllon, propose 21 new combinations, and provide a list of currently accepted taxa.