arXiv subjects

Elena Manjavacas

Publications and source records attributed to Elena Manjavacas.

At least 19 recordsLinked to original sources

arXiv2026

Multi-Epoch Stability in the Rotational Modulation of the Planetary-Mass Companion Ross 458C

Spectroscopic variability is detected in brown dwarfs across all spectral types, suggesting atmospheric patchiness in brown dwarf atmospheres. We present JWST/NIRSpec time series observations of the T8.0 planetary-mass companion Ross 458C. We compared our derived spectral variability and light curve to those obtained using HST/WFC3 observations more than 7.5 yr before and published by Manjavacas et al. (2019). We found that the light curve of Ross 458C is remarkably similar in the two epochs in terms of shape and variability amplitude potentially created by stable weather patterns. We measured a rotational period of 11.57+\-0.09 hr, demonstrating that the previously reported HST/WFC3 period of 6.75+\-1.58 hr is most likely a second-order harmonic produced by an unresolved double-peaked rotational modulation, likely due to the higher uncertainties of the HST/WFC3 light curve and the short time baseline of those observations. We measured the variability amplitude in the white light curve created using only the wavelength range covered by HST/WFC3 (1.10-1.63 micron), obtaining a value of 1.47+\-0.18%, which is consistent with that measured in the HST/WFC3 light curve.

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arXiv2026

A Grid of Contribution Functions to Help the Interpretation of Time-resolved Observations of Brown Dwarfs

We present a grid of thermal emission contribution functions and their corresponding atmospheric spectra model for cloudy and cloud-free brown dwarfs, spanning effective temperatures $T_{\rm eff} \in [500, 2000]$ K, surface gravities $\log g = [3.5, 5.5]$ [cgs], sedimentation efficiencies $f_{\rm sed} = 1, 2, 3, 4$ and 8, and eddy diffusion coefficients $\log K_{\rm zz} = [6, 11]$. We use temperature-pressure profiles from the Sonora Diamondback models in conjunction with the picaso radiative transfer code and the cloud modeling code virga, to explore how different atmospheric parameters influence the vertical distribution of emitted radiation for atmospheres in chemical equilibrium. We confirm that cloud opacity significantly impacts atmospheric emission depth in prominent molecular features such as CH$_4$, H$_2$O, CO, NH$_3$, CO$_2$, and atomic features Na and K. The atmospheric emission depth varies most notably in the $J$-band, and at mid-resolution spectra in the Na and K doublets. Our analysis highlights pressure-wavelength differences between cloudy and cloud-free atmospheres for characterizing L, L/T transition, and T-type brown dwarfs and demonstrates how contribution functions can guide targeted spectroscopic investigations. To facilitate observational planning, we provide an accessible database and web-based application that enable the community to compare observational data with our model grid of spectra and contribution functions with a wavelength range of 0.5 - 30 $μ$m.

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arXiv2026

The Dyn-Atmo Survey: High-contrast Imaging Spectroscopy of the Substellar Companion HD 13724 B with the JWST NIRSpec IFU

We present the first spectral analysis from the JWST Cycle 3 GO Program #6362, the Dyn-Atmo Survey of dynamical-atmospheric benchmarks, characterizing the atmospheres of directly-imaged companions with dynamical masses and measured orbits. This program has obtained moderate-resolution spectra (R$\sim$2700, 3-5$μ$m) using the NIRSpec/G395H IFU. We observe the T4-dwarf companion, HD 13724 B, companion to a G-type star at a projected angular separation of 0''2. Updated orbital monitoring constrains the mass to $38.3\pm0.6~M_{\rm{Jup}}$, which we use as a Gaussian prior when inferring the atmospheric state. We fit the continuum-subtracted spectrum using the BT-SETTL, Sonora Diamondback, Sonora Flame Skimmer, and NEWERA-PHOENIX self-consistent model grids, together with petitRADTRANS atmospheric retrievals. We adopt bulk parameter values of $T_\mathrm{eff}$=$1150\pm$195 K, $\log g$=5.1$\pm0.3$ dex, [M/H]=0.4$\pm$0.4, $\log_{10}(K_{zz})$=6.33$_{-2.22}^{+2.59}$, and C/O=$0.50\pm0.13$ using a bayesian average across model grids. We find degeneracies in the models between surface gravity, metallicity, and $K_{zz}$ for Sonora Diamondback and Flame Skimmer. The Flame Skimmer and NEWERA-PHOENIX vertical mixing strengths are incompatible, with Flame Skimmer finding a systematically lower value. The retrievals favor an atmosphere in chemical equilibrium, with enhanced metallicity and a solar C/O ratio, but with no detection of disequilibrium chemistry, and no detection of trace gases other than CH$_4$, CO$_{2}$, $^{12}$CO and H$_{2}$O. Ultimately, we identify critical challenges in forward modeling continuum-subtracted G395H spectra in the low signal-to-noise regime. We find that including independent dynamical mass measurements better constrains atmospheric parameters, but degeneracies between self-consistent model parameters limit accurate atmospheric parameter inferences.

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arXiv2026

The Dyn-Atmo Survey: JWST/NIRSpec spectroscopy of dynamical benchmark GJ 758 B

We present new JWST/NIRSpec IFU high contrast spectroscopic observations of the brown dwarf companion GJ 758 B. Extensive radial velocity monitoring of the primary has enabled high-precision measurements of the companion mass through characterization of their joint orbital dynamics. The combination of prior space- and ground-based photometry and spectroscopic observations in concert with a spectral model-independent mass makes this target an ideal benchmark for calibrating evolutionary models with known mass-age-luminosity degeneracies. A template-matching analysis on the NIRSpec/IFU data (F290LP,G395H; $λ\in$ [2.87-5.27] $μ$m) detects the companion at the expected orbital position with incredible significance (265.2$σ$). Comprehensive spectral analysis including published datasets over a variety of different spectral model families reveal that an inference using a uniform radius prior is plagued by inaccurate posteriors. The atmospheric model degeneracy between metallicity, gravity, and clouds pull the inferred mass/gravity/radius to an unphysical portion of parameter space for all model families tested. A constrained inference with a gaussian prior on the mass can mitigate this issue but models without clouds still struggle to accurately reproduce the K-band photometry and result in mass inferences which are inconsistent with the dynamics. We explore modifying the spectral models with custom post-processed clouds and molecular abundance perturbations of ammonia and hydrogen sulfide. We find the best-fitting model includes clouds with no patchiness (hole fraction f$_\mathrm{hole}$ = 0), a small fractional depletion in ammonia and moderate enhancement in hydrogen sulfide.

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arXiv2026

JWST color sequence of transiting exoplanets and directly imaged substellar objects

Color-magnitude diagrams (CMDs) have been foundational across astrophysics, from galaxies and stars to brown dwarfs, and JWST now extends them to transiting exoplanets and self-luminous substellar objects at overlapping temperatures. We compile JWST dayside emission spectra for 13 transiting giant planets, 57 self-luminous objects, and the irradiated brown dwarf ZTF J0038+2030 B, spanning ~350-2600 K and log g = 2.5-5.5, plus ~2150 SPHEREx ultracool dwarfs, converted to synthetic photometry in 2MASS J/Ks and five NIRCam medium bands isolating H2O, CH4, CO2, and CO. Low gravity and inflated radii make transiting planets resemble young substellar objects, with shallower features and redder colors from high-altitude clouds that raise the photosphere. Transiting planets follow the L-dwarf sequence on the J versus J-K CMD, with WASP-80 b (T_eq~800 K) showing no T-dwarf-like blueward turn, suggesting a delayed or suppressed L/T transition in irradiated, low-gravity atmospheres. Methane onset is delayed from substellar to transiting-planet atmospheres, and cloudy radiative-convective models indicate that lower gravity and cloud back-warming shift atmospheres toward CO-dominated chemistry, suppressing photospheric CH4. The irradiated but old, high-gravity brown dwarf ZTF J0038+2030 B (T_day = 1049 K, log g = 5.4) shows a deep, field-T-dwarf-like dayside methane band, implicating gravity rather than irradiation as the dominant control. CO2-to-CO diagnostics place transiting and directly imaged planetary-mass companions at stronger relative CO2 absorption than field brown dwarfs, consistent with metallicity enhancement from planetesimal accretion. These CMD sequences provide a unified, model-testable map for self-luminous and irradiated atmospheres.

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arXiv2026

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems VIII: patchy forsterite and enstatite clouds in the atmosphere of VHS 1256 b, retrieval lessons learned and outlook to the future

JWST defines a new era for the data-driven approach of retrieval modelling, which has become a cornerstone tool for the statistical inference of exoplanetary and brown dwarf properties. The Early Release Science program #1386 observations of VHS 1256 b represent a huge jump in data quality, data quantity and spectral coverage for such objects. VHS 1256 b is a young, planetary mass and extremely variable companion that populates the enigmatic L/T cohort of substellar atmospheres. In this first retrieval analysis of the full 1 - 18 micron dataset, we apply the Brewster retrieval framework to the NIRSpec and MIRI spectroscopic observations of VHS 1256 b, exploring a variety of cloud species and structures. Using Delta(BIC) we find that the data is best described by a forsterite (Mg$_{2}$SiO$_{4}$) and enstatite (MgSiO$_{3}$) cloud combination. Our analysis shows a strong preference for patchy silicate cloud coverage, which aligns with VHS 1256 b's extensive and well documented spectral variability. Our retrieval is able to place constraints on the abundances of H$_{2}$O, CO, CO$_{2}$, CH$_{4}$ as well as NH$_{3}$. We also show that the retrieved parameters are sensitive to the data used and the relative signal-to-noise ratios between data from different instruments. We conclude with the next steps for the wider retrieval community to better understand young and cloudy exoplanetary atmospheres.

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arXiv2026

Panchromatic View of the Frigid Jovian Exoplanet COCONUTS-2 b

We use a high signal-to-noise MIRI-LRS spectrum (5.45 - 11 $μ$m, R$_λ$ $\sim100$) of COCONUTS-2~b revealing prominent molecular features of H$_2$O, CH$_4$ and NH$_3$. This dataset is combined with spectra from Gemini/FLAMINGOS-2 and JWST/NIRSpec (G395H), as well as photometry from WISE and Spitzer, resulting in almost continuous wavelength coverage from 1 to 15 $μ$m. We analyze the data using five grids of self-consistent atmospheric models, spanning a wide range of T$_\text{eff}$, log(g), and [M/H]. We also investigate the use of Gaussian Processes to account for correlated noise either caused by the spectrograph or by systematic departures of models in the inversion framework. All models manage to fit the overall combined observations but predict fainter flux in Y- and N-bands. Classical model comparison suggests that the ATMO2020++ synthetic specra (with and without PH$_3$) are statistically preferred. Fitting for the correlated noise of the three spectroscopic instruments, ATMO2020++ models yields constraints consistent with previous studies and evolutionary models predictions: T$_\text{eff}$ $=496^{+5}_{-3}$ K, log(g) $=4.30^{+0.04}_{-0.02}$ dex, [M/H] $=-0.02^{+0.03}_{-0.02}$ dex, and R $=1.03^{+0.01}_{-0.02}$ R$_\text{jup}$. The extended wavelength coverage provided by MIRI (accounting for 41% of the bolometric flux) completes the SED, yielding a precise luminosity estimation of log(L/L$_{\odot}$) $=-6.166\pm0.002$ dex. Combined with a previous estimate of the system age ($414\pm23$ Myr), cooling models predict a mass of M $=7.3\pm0.3$ M$_\text{jup}$.

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arXiv2025

Large-amplitude Variability Driven by Giant Dust Storms on a Planetary-mass Companion

Large-amplitude variations are commonly observed in the atmospheres of directly imaged exoplanets and brown dwarfs. VHS 1256B, the most variable known planet-mass object, exhibits a near-infrared flux change of nearly 40%, with red color and silicate features revealed in recent JWST spectra, challenging current theories. Using a general circulation model, we demonstrate that VHS 1256B's atmosphere is dominated by planetary-scale dust storms persisting for tens of days, with large patchy clouds propagating with equatorial waves. This weather pattern, distinct from the banded structures seen on solar system giants, simultaneously explains the observed spectra and critical features in the rotational light curves, including the large amplitude, irregular evolution, and wavelength dependence, as well as the variability trends observed in near-infrared color-magnitude diagrams of dusty substellar atmospheres.

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arXiv2025

The JWST weather report from the nearest brown dwarfs III: Heterogeneous clouds and Thermochemical instabilities as possible drivers of WISE 1049AB's spectroscopic variability

We present a new analysis of the spectroscopic variability of WISE~J104915.57$-$531906.1AB (WISE~1049AB, L7.5+T0.5), observed using the NIRSpec instrument onboard the James Webb Space Telescope (GO 2965 - PI: Biller). We explored the variability of the dominant molecular bands present in their 0.6--5.3~$μ$m spectra (H$_2$O, CH$_4$, CO), finding that the B component exhibits a higher maximum deviation than the A component in all the wavelength ranges tested. The light curves reveal wavelength-(atmospheric depth) and possibly chemistry-dependent variability. In particular, for the A component, the variability in the light curves at the wavelengths traced by the CH$_4$ and CO molecular absorption features is higher than that of H$_2$O, even when both trace similar pressure levels. We concluded that clouds alone are unlikely to explain the increased variability of CO and CH$_4$ with respect to H$_2$O, suggesting that an additional physical mechanism is needed to explain the observed variability. This mechanism is probably due to thermochemical instabilities. Finally, we provide a visual representation of the 3D atmospheric map reconstructed for both components using the molecular band contributions at different pressure levels and the fit of planetary-scale waves.

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arXiv2025

Sensitivity to Sub-Io-sized Exosatellite Transits in the MIRI LRS Lightcurve of the Nearest Substellar Worlds

JWST's unprecedented sensitivity enables precise spectrophotometric monitoring of substellar worlds, revealing atmospheric variability driven by mechanisms operating across different pressure levels. This same precision now permits exceptionally sensitive searches for transiting exosatellites, small terrestrial companions to these worlds. Using a novel simultaneous dual-band search method to address host variability, we present a search for transiting exosatellites in an 8-hour JWST/MIRI LRS lightcurve of the nearby ($2.0\,pc$) substellar binary WISE J1049-5319AB, composed of two $\sim30 M_{\rm Jup}$ brown dwarfs separated by $3.5\,au$ and viewed near edge-on. Although we detect no statistically significant transits, our injection-recovery tests demonstrate sensitivity to satellites as small as $0.275\,R_{\oplus}$ ($0.96\,R_{\rm Io}$ or $\sim$1 lunar radius), corresponding to 300ppm transit depths, and satellite-to-host mass ratios $>$$10^{-6}$. This approach paves the way for detecting Galilean-moon analogs around directly imaged brown dwarfs, free-floating planets, and wide-orbit exoplanets, dozens of which are already scheduled for JWST lightcurve monitoring. In our Solar System, each giant planet hosts on average 3.5 moons above this threshold, suggesting that JWST now probes a regime where such companions are expected to be abundant. The technique and sensitivities demonstrated here mark a critical step toward detecting exosatellites and ultimately enabling constraints on the occurrence rates of small terrestrial worlds orbiting $1\text{-}70$$M_{\rm Jup}$ hosts.

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arXiv2025

A Near-Infrared Spectral Library of Very Young Brown Dwarfs and Planetary-Mass Objects in the Orion Nebula Cluster

Age-benchmark brown dwarfs' and planetary-mass objects' spectroscopy is key to characterize substellar evolution. In this paper we present the JHK medium resolution (R~3000) spectra of 25 7-75 M_Jup (spectral types L3.0-M6.0) brown dwarfs and planetary-mass objects in the Orion Nebula Cluster obtained with MOSFIRE installed at the W. M. Keck I telescope. We obtained the spectral types of the targets in our sample using template brown dwarf and planetary-mass objects' spectra. We confirmed their extreme youth (<5 Myr) and membership to the cluster using spectral indices, and the diversity of their spectra even for targets with similar spectral types. Six of our targets presented Pa-beta and Bra-gamma emission lines, suggesting the existence of accreting protoplanetary disks to objects with masses as low as 7 M_Jup. After analyzing the emission lines of those objects, and measuring their accretion rates, we compared them to those of stars, brown dwarfs and planetary-mass objects, confirming that planetary-mass young objects deplete their disks quickly at young ages. Finally, we illustrate the spectral evolution of a 7-10 M_Jup planetary-mass object through its life from 1-3 Myr to 200 Myr old using one of our latest spectra type targets, and other targets from the literature with older age, but similar estimated masses. The spectra are publicly available for the community's use as data behind the figures.

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arXiv2025

X-SHYNE: X-Shooter spectra of young exoplanet analogs II. Presentation and analysis of the full library

The X-SHYNE library is a homogeneous sample of 43 medium-resolution (R=8000) infrared (0.3-2.5um) spectra of young (<500Myr), low-mass (<20Mjup), and cold (Teff=600-2000K) isolated brown dwarfs and wide-separation companions observed with the VLT/X-Shooter instrument. To characterize our targets, we performed a global comparative analysis. We first applied a semi-empirical approach. By refining their age and bolometric luminosity, we derived key atmospheric and physical properties, such as Teff, mass, surface gravity (g), and radius, using the evolutionary model COND03. These results were then compared with the results from a synthetic analysis based on three self-consistent atmospheric models. To compare our spectra with these grids we used the Bayesian inference code ForMoSA. We found similar Lbol estimates between both approaches, but an underestimated Teff from the cloudy models, likely due to a lack of absorbers that could dominate the J and H bands of early L. We also observed a discrepancy in the log(g) estimates, which are dispersed between 3.5 and 5.5 dex for mid-L objects. We interpreted this as a bias caused by a range of rotational velocities leading to cloud migration toward equatorial latitudes, combined with a variety of viewing angles that result in different observed atmospheric properties (cloud column densities, atmospheric pressures, etc.). Finally, while providing robust estimates of [M/H] and C/O for individual objects remains challenging, the X-SHYNE library globally suggests solar values, which are consistent with a formation via stellar formation mechanisms. This study highlights the strength of homogeneous datasets in performing comparative analyses, reducing the impact of systematics, and ensuring robust conclusions while avoiding over-interpretation.

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arXiv2025

The JWST weather report from the nearest brown dwarfs II: Consistent variability mechanisms over 7 months revealed by 1-14 $μ$m NIRSpec + MIRI monitoring of WISE 1049AB

We present a new epoch of JWST spectroscopic variability monitoring of the benchmark binary brown dwarf WISE 1049AB, the closest, brightest brown dwarfs known. Our 8-hour MIRI low resolution spectroscopy (LRS) and 7-hour NIRSpec prism observations extended variability measurements for any brown dwarfs beyond 11 $μ$m for the first time, reaching up to 14 $μ$m. Combined with the previous epoch in 2023, they set the longest JWST weather monitoring baseline to date. We found that both WISE 1049AB show wavelength-dependent light curve behaviours. Using a robust k-means clustering algorithm, we identified several clusters of variability behaviours associated with three distinct pressure levels. By comparing to a general circulation model (GCM), we identified the possible mechanisms that drive the variability at these pressure levels: Patchy clouds rotating in and out of view likely shaped the dramatic light curves in the deepest layers between 1-2.5 $μ$m, whereas hot spots arising from temperature / chemical variations of molecular species likely dominate the high-altitude levels between 2.5-3.6 $μ$m and 4.3-8.5 $μ$m. Small-grain silicates potentially contributed to the variability of WISE 1049A at 8.5-11 $μ$m. While distinct atmospheric layers are governed by different mechanisms, we confirmed for the first time that each variability mechanism remains consistent within its layer over the long term. Future multi-period observations will further test the stability of variability mechanisms on this binary, and expanded JWST variability surveys across the L-T-Y sequence will allow us to trace and understand variability mechanisms across a wider population of brown dwarfs and planetary-mass objects.

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arXiv2025

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems III: Aperture Masking Interferometric Observations of the star HIP 65426 at 3.8 um

We present aperture masking interferometry (AMI) observations of the star HIP 65426 at $3.8\,\rm{μm}$ 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 $0.5λ/D$ 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 $5σ$ contrast of $Δm{\sim}7.62{\pm}0.13$ 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.

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arXiv2024

Hints of a sulfur-rich atmosphere around the 1.6 R$_{\oplus}$ Super-Earth L98-59 d from JWST NIRSpec G395H transmission spectroscopy

Detecting atmospheres around planets with a radius below 1.6 R$_{\oplus}$, commonly referred to as rocky planets (Rogers_2015, Rogers_2021), has proven to be challenging. However, rocky planets orbiting M-dwarfs are ideal candidates due to their favorable planet-to-star radius ratio. Here, we present one transit observation of the Super-Earth L98-59d (1.58 R$_{\oplus}$, 2.31 M$_{\oplus}$), at the limit of rocky/gas-rich, using the JWST NIRSpec G395H mode covering the 2.8 to 5.1 microns wavelength range. The extracted transit spectrum from a single transit observation deviates from a flat line by 2.6 to 5.6$σ$, depending on the data reduction and retrieval setup. The hints of an atmospheric detection are driven by a large absorption feature between 3.3 to 4.8 microns. A stellar contamination retrieval analysis rejected the source of this feature as being due to stellar inhomogeneities, making the best fit an atmospheric model including sulfur-bearing species, suggesting that the atmosphere of L98-59d may not be at equilibrium. This result will need to be confirmed by the analysis of the second NIRSpec G395H visit in addition to the NIRISS SOSS transit observation.

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arXiv2024

The Unrealised Interdisciplinary Advantage of Observing High Mass Transiting Exoplanets and Brown Dwarfs -- Strategic Exoplanet Initiatives with HST and JWST White Paper

We advocate for further prioritisation of atmospheric characterisation observations of high mass transiting exoplanets and brown dwarfs. This population acts as a unique comparative sample to the directly imaged exoplanet and brown dwarf populations, of which a range of JWST characterisation observations are planned. In contrast, only two observations of transiting exoplanets in this mass regime were performed in Cycle 1, and none are planned for Cycle 2. Such observations will: improve our understanding of how irradiation influences high gravity atmospheres, provide insights towards planetary formation and evolution across this mass regime, and exploit JWST's unique potential to characterise exoplanets across the known population.

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arXiv2024

JWST-TST High Contrast: Spectroscopic Characterization of the Benchmark Brown Dwarf HD 19467 B with the NIRSpec Integral Field Spectrograph

We present the atmospheric characterization of the substellar companion HD 19467 B as part of the pioneering JWST GTO program to obtain moderate resolution spectra (R$\sim$2,700, 3-5$μ$m) of a high-contrast companion with the NIRSpec IFU. HD 19467 B is an old, $\sim$9 Gyr, companion to a Solar-type star with multiple measured dynamical masses. The spectra show detections of CO, CO$_2$, CH$_4$, and H$_2$O. We forward model the spectra using Markov Chain Monte Carlo methods and atmospheric model grids to constrain the effective temperature and surface gravity. We then use NEWERA-PHOENIX grids to constrain non-equilibrium chemistry parameterized by $K_{zz}$ and explore molecular abundance ratios of the detected molecules. We find an effective temperature of 1103 K, with a probable range from 1000--1200 K, a surface gravity of 4.50 dex, with a range of 4.14--5.00, and deep vertical mixing, log$_{10}$($K_{zz}$), of 5.03, with a range of 5.00--5.44. All molecular mixing ratios are approximately Solar, leading to a C/O $\sim$0.55, which is expected from a T5.5 brown dwarf. Finally, we calculate an updated dynamical mass of HD 19467 B using newly derived NIRCam astrometry which we find to be $71.6^{+5.3}_{-4.6} M_{\rm{Jup}}$, in agreement with the mass range we derive from evolutionary models, which we find to be 63-75 $M_{\rm{Jup}}$.These observations demonstrate the excellent capabilities of the NIRSpec IFU to achieve detailed spectral characterization of substellar companions at high-contrast close to bright host stars, in this case at a separation of $\sim$1.6\arcsec with a contrast of 10$^{-4}$ in the 3-5 $μ$m range.

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arXiv2024

The JWST Weather Report from the Nearest Brown Dwarfs I: multi-period JWST NIRSpec + MIRI monitoring of the benchmark binary brown dwarf WISE 1049AB

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 $μ$m methane absorption feature and a tentative detection of small grain ($<$ 1$μ$m) silicate absorption at $>$8.5 $μ$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 $μ$m coincident with a CO absorption bandhead, 2) change in behavior at 4.2 $μ$m, close to the CO fundamental band at $λ>$ 4.4 $μ$m, and 3) change in behavior at 8.3-8.5 $μ$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 $μ$m and $>$8.5 $μ$m, 2) an intermediate pressure level shaping the lightcurve morphology between 2.3 and 4.2 $μ$m, and 3) a higher-altitude pressure level producing single-peaked and plateaued lightcurve behavior between 4.2 and 8.5 $μ$m.

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