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Channon Visscher

Publications and source records attributed to Channon Visscher.

At least 19 recordsLinked to original sources
arXiv2026

Time-resolved JWST Retrieval Analysis of the Coldest Brown Dwarf: Temperature and Chemical Variations in WISE 0855-07

WISE J0855-07 is the coldest known brown dwarf (estimated Teff~250 K), offering a rare opportunity to probe atmospheric physics in the temperature and mass regime that overlaps directly with exoplanets. At these temperatures, modest perturbations to the thermal structure and chemical abundances can produce measurable changes in the emergent mid-infrared spectrum, making time-resolved spectroscopy a powerful way to test whether the atmosphere is longitudinally and temporally homogeneous. Using James Webb Space Telescope NIRSpec/G395M time-series spectra spanning ~3 to 5.2 μm over 11 hr, we analyze every third spectrum from the dataset with a series of time-resolved atmospheric retrievals using the Brewster retrieval framework. Using this approach, we model epoch-to-epoch changes in the atmospheric state that drive the observed variability, including the thermal structure and key molecular abundances. From the time-series retrievals, we find that the primary source of the observed spectral variability is driven by the variations in the thermal structure, which range from ~1 to <100 K from epoch to epoch, particularly between the pressures of ~0.4 and 10 bar. In addition, changes in the abundances of CO, and possibly PH3 and CO2, further modulate the spectrum within specific molecular features. These measurements provide a retrieval-based view of atmospheric variability in an ultracold, planet-like object, directly constraining how thermal and chemical structure evolve on observable timescales.

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arXiv2026

Water Cloud and Chemical Modulations in the Coldest Brown Dwarf

We present high signal-to-noise, medium resolution (R ~ 1000), time-series JWST/NIRSpec spectra of WISE 0855 (265K), the coldest known brown dwarf. Medium resolution, time-series spectroscopy gives us the power to disentangle the effects of chemistry, temperature, and condensates on this cool world. Our observations span 11 hours with a 15 minute cadence covering 2.87 - 5.27 $μ$m. The strongest time variable spectroscopic feature is carbon monoxide gas absorption producing modulations with a peak-to-peak amplitude up to 10\% at some wavelengths. Using principal component analysis, we show that the variations in carbon monoxide and phosphine correlate with one another. By comparing our data to atmospheric and structure models we present evidence of patchy water clouds within the atmosphere of WISE 0855. We find that variations in CO and PH$_{3}$ abundances must originate from quenched atmospheric pressures while variations in water cloud thickness occurs at lower pressures.

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arXiv2026

Sinking Silicates I: Characterizing the benchmark system containing the T0 brown dwarf CWISE J210640.16+250729.0 using JWST

In this study, we present the full (97.3\% complete) 0.8--12.5 $μ$m spectral energy distribution (SED) of an L/T transition object, CWISE J210640.16+250729.0 (CW2106), using the James Webb Space Telescope (JWST). We provide a full characterization of the host star's elemental abundances and age. We empirically derive the bolometric luminosity ($L_{\rm bol}\approx-4.825$ $\textup{L}_\odot$) of CW2106, and obtain estimates of its mass (M$\approx50-62$ M$_{\rm Jup}$), radius (R$\approx0.83-0.87$ R$_{\rm Jup}$), effective temperature ($T_{\rm eff}$$\approx1213$ K), and surface gravity ($\log~g$$\approx5.28$ dex). We find the near-infrared (near-IR) spectrum ($0.8-2.5 ~μ$m) is best reproduced with cloudy atmospheric models while the mid-infrared (mid-IR) spectrum ($5-12.5 ~μ$m) is best reproduced with cloudless models. This suggests a cloud layer restricted to only the deepest observable parts of the atmosphere and is qualified by the lack of a 9 $μ$m silicate feature. Making use of the Mg/Si ratio of the primary, alongside thermochemical models, we predict the clouds in CW2106 to be composed primarily of enstatite (MgSiO$_3$), removing $\sim23\%$ of the bulk oxygen out of the atmosphere. Future retrieval studies will be able to help investigate the existence and full impact of these cloud species.

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arXiv2026

Benchmark Brown Dwarfs as Chemical Laboratories: Linking System Bulk Properties to Atmospheric Retrievals

We present the first atmospheric retrieval analysis of two compositional benchmark mid-L dwarfs -- SDSSJ141659.78+500626.4 and GJ 499 C -- using the \textit{Brewster} retrieval framework, where the wide benchmark nature of these systems provides independent constraints on age and bulk composition from their stellar primaries. These targets were observed with JWST using NIRSpec Prism and MIRI LRS, providing low-resolution ($R$ $\sim$ 100) spectra between 0.6--14.0 $\mathrmμ$m with high signal-to-noise ratios (SNR $\sim$100 -- 600). For SDSSJ141659.78+500626.4, the retrieved cloud combination of a high-altitude enstatite slab and low-altitude iron deck clouds matches phase-equilibrium predictions based on the primary star's Mg/Si ratio. We retrieve a super-solar C/O = 0.71$^{+0.01}_{-0.01}$ and slightly metal-rich [M/H] = 0.22$^{+0.03}_{-0.03}$. This C/O ratio can only be reconciled with the value inferred for the primary if additional oxygen sequestration beyond the retrieved cloud mass is present, or if there are uncertainties in the adopted opacities or other model deficiencies. The inferred [C/H] and [O/H] are 0.31$\pm$0.03 and 0.19$\pm$0.03, respectively, which are consistent within the relatively large uncertainties of the host star abundances. For GJ 499 C, the retrieved silicon-monoxide and forsterite slab clouds are difficult to explain with simple phase-equilibrium assumptions, yielding Mg/Si $\sim$ 1.9. We estimate C/O = 0.69$^{+0.02}_{-0.02}$ and [M/H] = 0.13$^{+0.04}_{-0.06}$. For both objects, the inferred radii of 0.85$^{+0.01}_{-0.01}$ $R_{\mathrm{Jup}}$ and 1.00$^{+0.02}_{-0.02}$ $R_{\mathrm{Jup}}$ and masses of 73.7$^{+4.9}_{-9.2}$ $M_{\mathrm{Jup}}$ and 68.0$^{+8.5}_{-12.7}$ $M_{\mathrm{Jup}}$ are consistent with evolutionary models and system ages, highlighting the plausibility of our results.

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arXiv2026

The Sonora Substellar Atmosphere Models. VII. Flame Skimmer: Cloud-free Atmospheric and Evolutionary Models for the Coldest Substellar Objects

JWST has provided unprecedented access to ultra-cool brown dwarfs and has pushed the boundaries of directly imaging temperate giant planets. As we continue to push toward detecting sub-Saturn and Neptune-like planets, it is crucial to develop atmospheric and evolutionary models that better capture the complexity and diversity of planetary atmospheres similar to the gas and ice giants in our Solar System. We present Sonora Flame Skimmer, the next suite of cloud-free 1D atmospheric and evolutionary models in chemical equilibrium and disequilibrium probing colder temperatures (down to 50 K), smaller objects (down to log(g) = 2), and a wide range of metallicities (10x sub-solar to 100x super-solar) and C/O ratios (solar to 2.5x solar). Beyond expanding the physical parameter space of previous Sonora models, we update the opacities and evolutionary model framework from Sonora Bobcat, as well as the chemical treatment of volatiles (H$_2$O, CH$_4$, NH$_3$) and carbon species such as CO$_2$ from Sonora Elf Owl. For the evolution of these substellar objects, we find that high-metallicity atmospheres lead to slower cooling compared to solar metallicity, while the strength of vertical mixing ($K_{\rm zz}$) has a negligible impact on the evolutionary tracks. At the highest metallicity explored here (100$\times$ solar), the deuterium-burning and hydrogen-burning minimum masses fall to 5.39 and 45.03 $M_{\rm J}$, respectively. All the models presented here, including the atmospheric structure, chemical profiles, spectra, synthetic photometry, and evolutionary models, are publicly available.

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arXiv2026

The JWST weather report: Unravelling the atmospheric variability of isolated worlds using principal component analysis

Brown dwarf variability directly probes atmospheric dynamics beyond the Solar System, and recent JWST time-resolved spectroscopy has opened a new window into these processes. Principal component analysis (PCA) offers a data-driven framework to identify the dominant, independent patterns of spectral variability of variable targets without relying on prior atmospheric assumptions. SIMP 0136 is a young, T2.5, brown dwarf at the planetary-mass boundary, making it an ideal analogue for directly imaged exoplanets. We analysed one rotation of JWST/NIRSpec PRISM time-series spectroscopy to investigate the drivers of its variability using PCA. Two principal components are sufficient to reduce the residual spectra to the propagated noise floor, indicating that they capture the detectable coherent spectroscopic variability. The leading principal component captures broadband variability consistent with temperature changes, while the second traces chromatic variability linked to vertical cloud structure. The dominance of two components implies that the spectra can be described as mixtures of three distinct atmospheric states, whose relative contributions we mapped as a function of rotational phase. The observed spectra are described as evolving linear combinations of these states, indicating that the variability arises from the changing visibility of spatially distinct atmospheric regions. By projecting Sonora Diamondback forward models into the same principal component space, we found that the principal components capture a large fraction of the model variance, demonstrating that the same physical processes that govern SIMP-0136's observed variability also capture much of the model grid's variation. Our results establish PCA as a computationally efficient, physically interpretable framework for analysing JWST time-resolved spectroscopy of substellar atmospheres.

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arXiv2026

Benchmark Brown Dwarf Systems I: Chemical Abundance Analysis of FGK Stars with Wide-Separation Brown Dwarf Companions Using PEPSI

We present results from a spectroscopic survey of 32 FGK stars hosting brown dwarfs, using high-resolution optical spectra (R = 130,000 and 50,000) obtained with the PEPSI spectrograph on the Large Binocular Telescope. The primary goal of this survey is to determine precise stellar parameters and abundances for 11 elements (C, O, Mg, Si, Ca, Al, Ti, Fe, Y, S, and N) in these systems. We employ spectral synthesis within the BACCHUS framework to derive precise stellar properties and elemental abundance ratios. For our average S/N $>$ 200 data, we achieve a typical error of 42 K in T$_\mathrm{eff}$ and $\sim$0.03 dex for [Fe/H]. We observe a significant dispersion from a solar C/O ratio among the sample of brown dwarf host stars that host primarily wide-orbit brown dwarfs. Using established theoretical chemical frameworks, we discuss the implications of the observed Mg/Si and Ca/Al ratios for cloud properties in the brown dwarf companions. Finally, we evaluate the applicability of the [Y/Mg] stellar clock for our sample and discuss the broader implications of our results. This work provides a timely and uniform abundance analysis of host stars, supporting extended wavelength brown dwarf observations in the era of JWST.

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arXiv2026

A Comprehensive Atmospheric Retrieval Analysis of 22 James Webb Space Telescope Spectral Energy Distributions of Cool Brown Dwarfs

We present a uniform atmospheric retrieval analysis of 22 late-T and Y-type brown dwarfs within 20 pc, observed with the James Webb Space Telescope NIRSpec PRISM and MIRI LRS. This dataset provides the first continuous 0.95-12 um spectroscopic coverage of late-T and Y-type brown dwarfs, which in turn enables precise constraints on their thermal structures and volume mixing ratios (VMRs) of H2O, CH4, CO, CO2, NH3, H2S, K, Na, and PH3. We find positive correlations between the VMR of H2O and CH4, and CO and CO2, consistent with thermochemical equilibrium chemistry. Using the VMRs, we derive atmospheric metallicity, which is positively correlated with H2O and CH4, showing H2O and CH4 trace oxygen and carbon content, respectively, allowing us to effectively measure (O/H)bulk and (C/H)bulk. We also report tentative PH3 detections in roughly half the sample, suggesting potential vertical mixing or non-equilibrium chemistry. Apart from chemical properties, we retrieve masses and radii spanning approximately 6-77 M_Jup and 0.66-1.53 R_Jup, respectively. We compare the derived log10(g) values of about 4-5.5 cm s^-2 and Teff values of about 350-1100 K with Sonora Bobcat evolutionary models and find an age range of 0.4 to 10 Gyr across the sample. Comparing our retrieved thermal profiles with the Elf-Owl forward-model thermal profiles, we find a systematic difference between the two, likely arising from differences in chemistry treatment.

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arXiv2026

PICASO 4.0: Clouds and Photochemistry in Climate Models of Brown Dwarfs and Exoplanets

We present a major update to the open-source atmospheric modeling package \texttt{PICASO}, designed for simulating the thermal structure and spectra of hydrogen-rich atmospheres of brown dwarfs and exoplanets. This release, \texttt{PICASO 4.0}, expands upon the existing radiative-convective equilibrium model framework by incorporating several new capabilities. Key additions include the integration of \texttt{Virga} for self-consistent cloud modeling, new flexible treatments for rainout and cold trapping of volatile species, and support for photochemistry. We also introduce a parameterized energy injection scheme to simulate additional external or internal heating processes. These features are motivated by lessons from recent JWST observations that reveal the prevalence of non-equilibrium chemistry and clouds. We benchmark the new functionalities against previously published results in the literature, including the Sonora Diamondback grid, energy injected atmospheres, patchy cloud models, and other photochemical models of WASP-39b. \texttt{PICASO} continues to be actively developed as an open-source package aimed at enabling reproducible, community-driven atmospheric modeling of all substellar objects.

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arXiv2026

Bridging the Gap: Using Brown Dwarfs to Examine Silicate Clouds in Giant Exoplanet Atmospheres

We present results from examining the silicate cloud modeling of four JWST-observed hot Jupiters in the context of brown dwarf theory to further explore signatures of formation in present-day atmospheres. We contextualize our understanding of protoplanetary disk refractory chemistry with empirical evidence from chondritic meteorites to show that giant planets forming and accreting in the outer disk adopt their stellar Mg/Si value. We show that current silicate cloud species determinations of WASP-17 b, WASP-107 b, WASP-39 b and HD 189733 b are in line with predictions laid out in Calamari et al. 2024 based on each system's host star Mg/Si ratio, further supporting this hypothesis. We discuss physical motivations for potential atmospheric scenarios where apparent silicate cloud species is not in agreement with that predicted by its host star chemistry. Additionally, we compare current transit spectroscopy for three of these four exoplanets against brown dwarf spectra to examine molecular absorption trends across the substellar mass temperature regime.

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arXiv2026

Mapping the Cloud-Driven Atmospheric Dynamics & Chemistry of an Isolated Exoplanet Analog with Harmonic Signatures

Young planetary-mass objects and brown dwarfs near the L/T spectral transition exhibit enhanced spectrophotometric variability over field brown dwarfs. Patchy clouds, auroral processes, stratospheric hot spots, and complex carbon chemistry have all been proposed as potential sources of this variability. Using time-resolved, low-to-mid-resolution spectroscopy collected with the JWST/NIRISS and NIRSpec instruments, we apply harmonic analysis to SIMP J013656.5+093347, a highly variable, young, isolated planetary-mass object. Odd harmonics (k = 3) at pressure levels ~ 1 bar, corresponding to iron and forsterite cloud formation, suggest a potential North-South hemispheric asymmetry in the cloudy, and likely equatorial, regions. We use the inferred harmonics, along with 1-D substellar atmospheric models, to map the flux variability by atmospheric pressure level. We identify distinct time-varying structures in the near-infrared that we interpret as planetary-scale wave (e.g., Rossby or Kelvin)-associated cloud modulation. We detect deviations from bulk (composite) variability in water (S/N = 14.0), carbon monoxide (S/N = 13.0), and methane (S/N = 14.9) molecular signatures. Forsterite cloud modulation is anti-correlated with overlying carbon monoxide and water abundances and correlated with deep methane absorption, suggesting complex interaction between cloud formation, atmospheric chemistry, and temperature structure. Furthermore, we identify distinct harmonic behavior between methane and carbon monoxide absorption bands, providing evidence for time-resolved disequilibrium carbon chemistry. At the lowest pressures (< 100 mbar), we find mapped methane lines transition from absorption to emission, supporting evidence of high-altitude auroral heating via electron precipitation.

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arXiv2025

Observation of undepleted phosphine in the atmosphere of a low-temperature brown dwarf

The atmospheres of low-temperature brown dwarfs and gas giant planets are expected to contain the phosphine molecule, PH$_3$ However, previous observations have shown much lower abundances of this molecule than predicted by atmospheric chemistry models. We report JWST spectroscopic observations of phosphine in the atmosphere of the brown dwarf Wolf 1130C. Multiple absorption lines due to phosphine are detected around 4.3 $μ$m, from which we calculate a phosphine abundance of 0.100$\pm$0.009 parts per million. This abundance is consistent with disequilibrium atmospheric chemistry models that reproduce the phosphine abundances in Jupiter and Saturn, and is much higher than abundances previously reported for other brown dwarfs or exoplanets.

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arXiv2025

The JWST Weather Report: retrieving temperature variations, auroral heating, and static cloud coverage on SIMP-0136

SIMP-0136 is a T2.5 brown dwarf whose young age ($200\pm50$~Myr) and low mass ($15\pm3$~M$_{\rm Jup}$) make it an ideal analogue for the directly imaged exoplanet population. With a 2.4 hour period, it is known to be variable in both the infrared and the radio, which has been attributed to changes in the cloud coverage and the presence of an aurora respectively. To quantify the changes in the atmospheric state that drive this variability, we obtained time-series spectra of SIMP-0136 covering one full rotation with both NIRSpec/PRISM and the MIRI/LRS on board JWST. We performed a series of time-resolved atmospheric retrievals using petitRADTRANS in order to measure changes in the temperature structure, chemistry, and cloudiness. We inferred the presence of a ~250 K thermal inversion above 10 mbar of SIMP-0136 at all phases, and propose that this inversion is due to the deposition of energy into the upper atmosphere by an aurora. Statistical tests were performed in order to determine which parameters drive the observed spectroscopic variability. The primary contribution was due to changes in the temperature profile at pressures deeper than 10 mbar, which resulted in variation of the effective temperature from 1243 K to 1248 K. This changing effective temperature was also correlated to observed changes in the abundances of CO2 and H2S, while all other chemical species were consistent with being homogeneous throughout the atmosphere. Patchy silicate clouds were required to fit the observed spectra, but the cloud properties were not found to systematically vary with longitude. This work paints a portrait of an L/T transition object where the primary variability mechanisms are magnetic and thermodynamic in nature, rather than due to inhomogeneous cloud coverage.

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arXiv2025

Silicate Precursor Silane detected in Cold Low-Metallicity Brown Dwarf

Within 20 pc of the Sun there are currently 29 known cold brown dwarfs, sources with measured distances and an estimated effective temperature between that of Jupiter (170K) and ~500K. These sources are almost all isolated and are the closest laboratories we have for detailed atmospheric studies of giant planets formed outside the solar system. Here we report JWST observations of one such source, WISEA J153429.75-104303.3 (W1534), which we confirm is a substellar mass member of the Galactic halo with a metallicity <0.01xsolar. Its spectrum reveals methane (CH4), water (H2O), and silane (SiH4) gas. Although SiH4 is expected to serve as a key reservoir for the cloud-forming element Si in gas giant worlds, it eluded detection until now because it is removed from observable atmospheres by the formation of silicate clouds at depth. These condensates are favored with increasing metallicity, explaining why SiH4 remains undetected on well studied, metal-rich solar system worlds like Jupiter and Saturn. On the metal-poor world W1534, we detect a clear signature of SiH4 centered at ~4.55 microns with an abundance of 19+/-2 parts per billion (ppb). Our chemical modelling suggests that this SiH4 abundance may be quenched at ~kilobar levels just above the silicate cloud layers, whereupon vertical atmospheric mixing can transport SiH4 to the observable photosphere. The formation and detection of SiH4 demonstrates key coupled relationships between composition, cloud formation, and atmospheric mixing in cold brown dwarf and planetary atmospheres.

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arXiv2025

Condensation Clouds in Substellar Atmospheres with Virga

Here we present an open-source cloud model for substellar atmospheres, called Virga. The Virga-v0 series has already been widely adopted in the literature. It is written in Python and has heritage from the Ackerman & Marley (2001) model (often referred to as eddysed), used to study clouds on both exoplanets and brown dwarfs. In the development of the official Virga-v1 we have retained all the original functionality of eddysed and updated/expanded several components including the back-end optical constants data, calculations of the Mie properties, available condensate species, saturation vapor pressure curves and formalism for fall speeds calculations. Here we benchmark Virga by reproducing key results in the literature, including the SiO2 cloud detection in WASP-17 b and the brown dwarf Diamondback-Sonora model series. Development of Virga is ongoing, with future versions already planned and ready for release. We encourage community feedback and collaborations within the GitHub code repository.

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arXiv2025

The Sonora Substellar Atmosphere Models. V: A Correction to the Disequilibrium Abundance of CO$_2$ for Sonora Elf Owl

To aid the interpretation of observations of substellar atmospheres, Mukherjee et al. (2024) created the Sonora Elf Owl grid of model atmospheres, simulations that accounted for disequilibrium quench chemistry. However, Sonora Elf Owl did not accurately estimate CO$_2$ quenching because the models quenched the gas with respect to the full atmosphere equilibrium, but CO$_2$ should have instead been quenched with respect to the disequilibrium (i.e., quenched) abundance of CO. As a result, Sonora Elf Owl under-predicted the CO$_2$ abundance by several order of magnitude in some instances, an amount that JWST is sensitive to. Here, we release version two of the Sonora Elf Owl grid which has corrected CO$_2$ concentrations. Additionally, in version two we remove PH$_3$ as a spectral contributor since our spectra consistently contained too much PH$_3$ absorption. The new spectra can be found as an update to the original Zenodo postings.

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arXiv2024

Protosolar D-to-H abundance and one part-per-billion PH$_{3}$ in the coldest brown dwarf

The coldest Y spectral type brown dwarfs are similar in mass and temperature to cool and warm ($\sim$200 -- 400 K) giant exoplanets. We can therefore use their atmospheres as proxies for planetary atmospheres, testing our understanding of physics and chemistry for these complex, cool worlds. At these cold temperatures, their atmospheres are cold enough for water clouds to form, and chemical timescales increase, increasing the likelihood of disequilibrium chemistry compared to warmer classes of planets. JWST observations are revolutionizing the characterization of these worlds with high signal-to-noise, moderate resolution near- and mid-infrared spectra. The spectra have been used to measure the abundances of prominent species like water, methane, and ammonia; species that trace chemical reactions like carbon monoxide; and even isotopologues of carbon monoxide and ammonia. Here, we present atmospheric retrieval results using both published fixed-slit (GTO program 1230) and new averaged time series observations (GO program 2327) of the coldest known Y dwarf, WISE 0855-0714 (using NIRSpec G395M spectra), which has an effective temperature of $\sim$ 264 K. We present a detection of deuterium in an atmosphere outside of the solar system via a relative measurement of deuterated methane (CH$_{3}$D) and standard methane. From this, we infer the D/H ratio of a substellar object outside the solar system for the first time. We also present a well-constrained part-per-billion abundance of phosphine (PH$_{3}$). We discuss our interpretation of these results and the implications for brown dwarf and giant exoplanet formation and evolution.

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arXiv2024

The JWST Weather Report from the Isolated Exoplanet Analog SIMP 0136+0933: Pressure-Dependent Variability Driven by Multiple Mechanisms

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.

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