Explore the vast range of titles available.

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 H 3 + and methane. Isolated brown dwarfs with auroral signatures in the radio have been searched for corresponding infrared features, but only null detections have been reported 3 . 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 H 3 + emission, which is prominent in Solar System gas giants. However, this is consistent with rapid destruction of H 3 + at the higher pressure where the W1935 emission originates 4 . Methane emission from a very cool brown dwarf, perhaps arising from an aurora, has been detected in James Webb Space Telescope observations.

Analysis of all archival 5--14 micron spectra of field ultracool dwarfs from the Infrared Spectrograph on the Spitzer Space Telescope has shown that absorption by silicates in the 8--11 micron region is seen in most L-type (1300 K to 2200 K) dwarfs. The absorption is caused by silicate-rich clouds in the atmospheres of L dwarfs and is strongest at L4--L6 spectral types. Herein we compare averages of the mid-infrared silicate absorption signatures of L3--L7 dwarfs that have low (\\(\\lesssim\\)10\\(^{4.5}\\) cm s\\(^{-2}\\)) vs.\\ high (\\(\\gtrsim\\)10\\(^5\\) cm s\\(^{-2}\\)) surface gravity. We find that the silicate absorption feature is sensitive to surface gravity and indicates a difference in grain size and composition between dust condensates in young and old mid-L dwarfs. The mean silicate absorption profile of low-gravity mid-L dwarfs matches expectations for \\(\\sim\\)1 micron-sized amorphous iron- and magnesium-bearing pyroxene (Mg\\(_x\\)Fe\\(_{1-x}\\)SiO\\(_3\\)) grains. High-gravity mid-L dwarfs have silicate absorption better represented by smaller (\\(\\lesssim\\)0.1 \\(\\mu\\)m) and more volatile amorphous enstatite (MgSiO\\(_3\\)) or SiO grains. This is the first direct spectroscopic evidence for gravity-dependent sedimentation of dust condensates in ultracool atmospheres. It confirms theoretical expectations for lower sedimentation efficiencies in low-gravity atmospheres and independently confirms their increased dustiness.

We report results from 8 hours of JWST/MIRI LRS spectroscopic monitoring directly followed by 7 hours of JWST/NIRSpec prism spectroscopic monitoring of the benchmark binary brown dwarf WISE 1049AB, the closest, brightest brown dwarfs known. We find water, methane, and CO absorption features in both components, including the 3.3 \\(\\mu\\)m methane absorption feature and a tentative detection of small grain (\\(<\\) 1\\(\\mu\\)m) silicate absorption at \\(>\\)8.5 \\(\\mu\\)m in WISE 1049A. Both components vary significantly (\\(>\\)1\\(\\%\\)), with WISE 1049B displaying larger variations than WISE 1049A. Using K-means clustering, we find three main transition points in wavelength for both components of the binary: 1) change in behavior at \\(\\sim\\)2.3 \\(\\mu\\)m coincident with a CO absorption bandhead, 2) change in behavior at 4.2 \\(\\mu\\)m, close to the CO fundamental band at \\(\\lambda >\\) 4.4 \\(\\mu\\)m, and 3) change in behavior at 8.3-8.5 \\(\\mu\\)m, potentially corresponding to silicate absorption. We interpret the lightcurves observed with both NIRSpec and MIRI as likely stemming from 1) a deep pressure level driving the double-peaked variability seen in WISE 1049B at wavelengths \\(<\\)2.3 \\(\\mu\\)m and \\(>\\)8.5 \\(\\mu\\)m, 2) an intermediate pressure level shaping the lightcurve morphology between 2.3 and 4.2 \\(\\mu\\)m, and 3) a higher-altitude pressure level producing single-peaked and plateaued lightcurve behavior between 4.2 and 8.5 \\(\\mu\\)m.

We present a uniform analysis of all mid-infrared \\(R\\approx90\\) spectra of field M5--T9 dwarfs obtained with the Spitzer Infrared Spectrograph (IRS). The sample contains 113 spectra out of which 12 belong to late-M dwarfs, 69 to L dwarfs, and 32 to T dwarfs. Sixty-eight of these spectra are presented for the first time. We measure strengths of the main absorption bands in the IRS spectra, namely H\\(_2\\)O at 6.25 \\(\\mu\\)m, CH\\(_4\\) at 7.65 \\(\\mu\\)m, NH\\(_3\\) at 10.5 \\(\\mu\\)m, and silicates over 8--11 \\(\\mu\\)m. Water absorption is present in all spectra and strengthens with spectral type. The onset of methane and ammonia occurs at the L8 and T2.5 types, respectively, although ammonia can be detectable as early as T1.5. Silicate absorption sets in at spectral type L2, is on average the strongest in L4--L6 dwarfs, and disappears past L8. However, silicate absorption can also be absent from the spectra at any L subtype. We find a positive correlation between the silicate absorption strength and the excess (deviation from median) near-infrared colour at a given L subtype, which supports the idea that variations of silicate cloud thickness produce the observed colour scatter in L dwarfs. We also find that variable L3--L7 dwarfs are twice more likely to have above-average silicate absorption than non-variables. The ensemble of results solidifies the evidence for silicate condensate clouds in the atmospheres of L dwarfs, and for the first time observationally establishes their emergence and sedimentation between effective temperatures of \\(\\approx\\)2000 K and \\(\\approx\\)1300 K, respectively.

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.

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)_{\\rm MKO}\\) color of 2.97\\(\\pm\\)0.03 mag and a \\(J_{\\rm MKO}-\\)W2 color of 4.93\\(\\pm\\)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\\(\\gamma\\)-T0\\(\\gamma\\). We tentatively find that CWISE J0506\\(+\\)0738 is variable at 3-5 \\(\\mu\\)m based on multi-epoch 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 \\(\\beta\\) Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object.

Isolated planetary-mass objects share their mass range with planets but do not orbit a star. They lack the necessary mass to support fusion in their cores and thermally radiate their heat from formation as they cool, primarily at infrared wavelengths. Many isolated planetary-mass objects show variations in their infrared brightness consistent with non-uniform atmospheric features modulated by their rotation. SIMP J013656.5+093347.3 is a rapidly rotating isolated planetary-mass object, and previous infrared monitoring suggests complex atmospheric features rotating in and out of view. The physical nature of these features is not well understood, with clouds, temperature variations, thermochemical instabilities, and infrared-emitting aurora all proposed as contributing mechanisms. Here we report JWST time-resolved low-resolution spectroscopy from 0.8 - 11 micron of SIMP J013656.5+093347.3 which supports the presence of three specific features in the atmosphere: clouds, hot spots, and changing carbon chemistry. We show that no single mechanism can explain the variations in the time-resolved spectra. When combined with previous studies of this object indicating patchy clouds and aurorae, these measurements reveal the rich complexity of the atmosphere of SIMP J013656.5+093347.3. Gas giant planets in the solar system, specifically Jupiter and Saturn, also have multiple cloud layers and high-altitude hot spots, suggesting these phenomena are also present in worlds both within and beyond our solar-system.

We present aperture masking interferometry (AMI) observations of the star HIP 65426 at \\(3.8\\,\\rm{\\mu m}\\) as a part of the JWST Direct Imaging Early Release Science (ERS) program obtained using the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument. This mode provides access to very small inner working angles (even separations slightly below the Michelson limit of \\(0.5\\lambda/D\\) for an interferometer), which are inaccessible with the classical inner working angles of the JWST coronagraphs. When combined with JWST's unprecedented infrared sensitivity, this mode has the potential to probe a new portion of parameter space across a wide array of astronomical observations. Using this mode, we are able to achieve a \\(5\\sigma\\) contrast of \\(\\Delta m{\\sim}7.62{\\pm}0.13\\) mag relative to the host star at separations \\({\\gtrsim}0.07{\"}\\), and the contrast deteriorates steeply at separations \\({\\lesssim}0.07{\"}\\). However, we detect no additional companions interior to the known companion HIP 65426 b (at separation \\({\\sim}0.82{\"}\\) or, \\(87^{+108}_{-31}\\,\\rm{au}\\)). Our observations thus rule out companions more massive than \\(10{-}12\\,\\rm{M_{Jup}}\\) at separations \\({\\sim}10{-}20\\,\\rm{au}\\) from HIP 65426, a region out of reach of ground or space-based coronagraphic imaging. These observations confirm that the AMI mode on JWST is sensitive to planetary mass companions at close-in separations (\\({\\gtrsim}0.07{\"}\\)), even for thousands of more distant stars at \\(\\sim\\)100 pc, in addition to the stars in the nearby young moving groups as stated in previous works. This result will allow the planning and successful execution of future observations to probe the inner regions of nearby stellar systems, opening an essentially unexplored parameter space.

We present a performance analysis for the aperture masking interferometry (AMI) mode on board the James Webb Space Telescope Near Infrared Imager and Slitless Spectrograph (JWST/NIRISS). Thanks to self-calibrating observables, AMI accesses inner working angles down to and even within the classical diffraction limit. The scientific potential of this mode has recently been demonstrated by the Early Release Science (ERS) 1386 program with a deep search for close-in companions in the HIP 65426 exoplanetary system. As part of ERS 1386, we use the same data set to explore the random, static, and calibration errors of NIRISS AMI observables. We compare the observed noise properties and achievable contrast to theoretical predictions. We explore possible sources of calibration errors and show that differences in charge migration between the observations of HIP 65426 and point-spread function calibration stars can account for the achieved contrast curves. Lastly, we use self-calibration tests to demonstrate that with adequate calibration NIRISS F380M AMI can reach contrast levels of \\(\\sim9-10\\) mag at \\(\\gtrsim \\lambda/D\\). These tests lead us to observation planning recommendations and strongly motivate future studies aimed at producing sophisticated calibration strategies taking these systematic effects into account. This will unlock the unprecedented capabilities of JWST/NIRISS AMI, with sensitivity to significantly colder, lower-mass exoplanets than lower-contrast ground-based AMI setups, at orbital separations inaccessible to JWST coronagraphy.

We present \\(Spitzer\\) IRS 5--14 \\(\\mu\\)m spectra and 16 \\(\\mu\\)m and 22 \\(\\mu\\)m photometry of the T2.5 companion to the \\(\\sim\\)300 Myr-old G0V star HN Peg. We incorporate previous 0.8--5 \\(\\mu\\)m observations to obtain the most comprehensive spectral energy distribution of an intermediate-gravity L/T-transition dwarf which, together with an accurate Gaia EDR3 parallax of the primary, enable us to derive precise fundamental parameters. We find that young (\\(\\approx\\)0.1--0.3 Gyr) early-T dwarfs on average have \\(\\approx\\)140 K lower effective temperatures, \\(\\approx\\)20% larger radii, and similar bolometric luminosities compared to \\(\\gtrsim\\)1 Gyr-old field dwarfs with similar spectral types. Our accurate infrared spectrophotometry offers new detail at wavelengths where the dominant carbon-bearing molecules have their strongest transitions: at 3.4 \\(\\mu\\)m for methane and at 4.6 \\(\\mu\\)m for carbon monoxide. We assess the performance of various widely available photospheric models and find that models with condensates and/or clouds better reproduce the full SED of this moderately young early-T dwarf. However, cloud-free models incorporating a more general convective instability treatment reproduce at least the low-resolution near-IR spectrum similarly well. Our analysis of \\(R\\approx2300\\) \\(J\\)-band spectra shows that the near-infrared potassium absorption lines in HN Peg B have similar strengths to those seen in both younger and older T2-T3 dwarfs. We conclude that while alkali lines are well-established as surface gravity indicators for L-type or warmer stars, they are insensitive to surface gravity in early-T dwarfs