Theodora Karalidi

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 Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) observations more than 7.5 yr before and published by E. Manjavacas et al. 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 μ m), obtaining a value of 1.47% ± 0.18%, which is consistent with that measured in the HST/WFC3 light curve.

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 _eff ∈ [500, 2000][K], surface gravities log g = [ 3.5 , 5.5 ] [cgs], sedimentation efficiencies f _sed = 1, 2, 3, 4, and 8, and eddy diffusion coefficients log K 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 midresolution 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.

We present a near-infrared spectropolarimetric survey of brown dwarfs conducted with the WIRC+Pol instrument on the Palomar 200 inch Hale Telescope. The primary survey goal was to investigate the presence of polarization signatures arising from cloudy atmospheres in these objects, with possible asymmetries indicating patchy cloud distributions. Our sample includes 22 brown dwarfs spanning M6 to T2 spectral types, a crucial phase in the evolution of substellar objects characterized by suspected drastic changes in atmospheric composition and structure. We find that only three of the brown dwarfs in our sample show a detectable polarization signal: CFHT BD Tau 4 and 2MASS J04221413+1530525, which are both located in or near molecular clouds, complicating interpretation of the polarimetric signal due to likely contamination from interstellar dust scattering; and 2MASS J15074769-1627386, which is a nearby L dwarf with previously confirmed photometric variability and measured optical polarization, here with its first measured near-infrared polarization. For this target, we rule out interstellar polarization and rotationally induced oblateness as possible causes of the measured polarization, potentially suggesting atmospheric asymmetries as the culprit.

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 on board the James Webb Space Telescope (GO 2965; PI: Biller). We explore the variability of the dominant molecular bands present in their 0.6–5.3 μ m spectra (H ₂ O, CH ₄ , and 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-dependent (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 for of H ₂ O, even when both trace similar pressure levels. We conclude 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 visual representations of the 3D atmospheric maps reconstructed for both components using the molecular band contributions at different pressure levels and the fits of planetary-scale waves.

We present JWST NIRSpec/PRISM integral field unit time-resolved observations of 2M1207 A and b (TWA 27), an ∼10 Myr binary system consisting of an ∼2500 K substellar primary hosting an ∼1300 K companion. Our data provide 20 time-resolved spectra over an observation spanning 12.56 hr. We provide an empirical characterization for the spectra of both objects across time. For 2M1207 A, nonlinear trend models are statistically favored within the ranges 0.6–2.3 μ m and 3.8–5.3 μ m. However, most of the periods constrained from sinusoidal models exceed the observing window, setting a lower limit of 12.56 hr. We find the data at H α and beyond 4.35 μ m show a moderate time correlation, as well as a pair of light curves at 0.73–0.80 μ m and 3.36–3.38 μ m. For 2M1207 b, light curves integrated across 0.86–1.77 μ m and 3.29–4.34 μ m support linear trend models. Following the interpretation of Z. Zhang et al., we model the 2M1207 b data with two 1D atmospheric components, both with silicate and iron condensates. The model of time variability due to changes in the cloud filling factor shows broad consistency with the variability amplitudes derived from our data. Our amplitudes, however, disagree with the models at ≈0.86–1 μ m. While an additional model component such as rainout chemistry may be considered here, our analysis is limited by low signal-to-noise ratio. Our results demonstrate the capability of JWST to simultaneously monitor the spectral variability of a planetary-mass companion and host at low contrast.

Determining the habitability of terrestrial exoplanets is a complex problem that represents the next major step for the astrophysical community. The majority of current models treat these planets as homogeneous or contain heterogeneity that is constant in time. In reality, habitable exoplanets are expected to contain atmospheric and surface heterogeneities similar to Earth, with diurnal rotation, seasonal changes, and weather patterns resulting in complex, time-dependent signatures. Due to its ability to measure light as a vector, polarimetry provides an important tool that will enhance the characterizations of heterogeneous worlds. Here we model the visible to near-infrared linear spectropolarimetric signatures, as functions of wavelength and planetary phase angle, of various heterogeneous Earth scenarios as well as the first signals of an early wet and potentially habitable Mars. The contributions from the different atmospheric and surface properties result in asymmetric phase curves and variable spectra, with the polarization appearing to be more sensitive than flux to heterogeneities such as patchy clouds and continents moving into and out of view. Our models provide important predictions of expected polarized and unpolarized signatures of heterogeneous exoplanets that will help guide the designs and observing plans of future polarimeters, including those proposed for the upcoming Habitable Worlds Observatory.

The search for life beyond the solar system remains a primary goal of current and near-future missions, including NASA’s upcoming Habitable Worlds Observatory (HWO). However, research into determining the habitability of terrestrial exoplanets has been primarily focused on comparisons to modern-day Earth. Additionally, current characterization strategies focus on the unpolarized flux from these worlds, taking into account only a fraction of the informational content of the reflected light. Better understanding the changes in the reflected-light spectrum of the Earth throughout its evolution, as well as analyzing its polarization, will be crucial for mapping its habitability and providing comparison templates to potentially habitable exoplanets. Here we present spectropolarimetric models of the reflected light from the Earth at six epochs across all four geologic eons. We find that the changing surface albedos and atmospheric gas concentrations across the different epochs allow the habitable and nonhabitable scenarios to be distinguished, and diagnostic features of clouds and hazes are more noticeable in the polarized signals. We also discuss how using Mie scattering for naturally nonspherical particles, which is a common simplification for exoplanet modeling, affects the resulting planetary signals. Finally, our results suggest that pushing the HWO planet-to-star flux contrast limit down to 1 × 10⁻¹³ could allow for the characterization in both unpolarized and polarized light of an Earth-like planet at any stage in its history.

Located 10.888 pc from Earth, COCONUTS-2b is a planetary-mass companion to a young (150–800 Myr) M3 star, with a wide orbital separation (6471 au) and a low companion-to-host mass ratio (0.021 ± 0.005). We have studied the atmospheric properties of COCONUTS-2b using newly acquired 1.0–2.5 μm spectroscopy from Gemini/Flamingos-2. The spectral type of COCONUTS-2b is refined to T9.5 ± 0.5 based on comparisons with T/Y dwarf spectral templates. We have conducted an extensive forward-modeling analysis, comparing the near-infrared spectrum and mid-infrared broadband photometry of COCONUTS-2b with 16 state-of-the-art atmospheric model grids developed for brown dwarfs and self-luminous exoplanets near the T/Y transition. The PH ₃ -free ATMO2020++, ATMO2020++, and Exo-REM models best match the specific observations of COCONUTS-2b, regardless of variations in the input spectrophotometry. This analysis suggests the presence of disequilibrium chemistry, along with a diabatic thermal structure and/or clouds, in the atmosphere of COCONUTS-2b. All models predict fainter Y-band fluxes than observed, highlighting uncertainties in the alkali chemistry models and opacities. We determine a bolometric luminosity of log ( L bol / L ⊙ ) = − 6.18 dex, with a 0.5 dex wide range of [−6.43, −5.93] dex that accounts for various assumptions of atmospheric models. Using several thermal evolution models, we derive an effective temperature of T eff = 483 − 53 + 44 K, a surface gravity of log ( g ) = 4.19 − 0.13 + 0.18 dex, a radius of R = 1.11 − 0.04 + 0.03 R _Jup, and a mass of M = 8 ± 2 M _Jup. Various atmospheric model grids consistently indicate that COCONUTS-2b’s atmosphere likely has subsolar or near-solar metallicity and C/O. These findings provide valuable insights into COCONUTS-2b’s formation history and the potential outward migration to its current wide orbit.

Most brown dwarfs show some level of photometric or spectral variability. However, finding the most variable dwarfs more suited for a thorough variability monitoring campaign remained a challenge until a few years ago with the design of spectral indices to find the most likely L and T dwarfs using their near-infrared (NIR) single-epoch spectrum. In this work, we designed and tested NIR spectral indices to preselect the most likely variable L4–L8 dwarfs, complementing the indices presented by Ashraf et al. and Oliveros-Gomez et al. We used time-resolved NIR Hubble Space Telescope Wide Field Camera 3 spectra of an L6.0 dwarf, LP 261–75b, to design our novel spectral indices. We tested these spectral indices on 75 L4.0–L8.0 NIR SpeX/IRTF spectra, providing 27 new variable candidates. Our indices have a recovery rate of ∼80% and a false negative rate of ∼25%. All the known nonvariable brown dwarfs were found to be nonvariable by our indices. We estimated the variability fraction of our sample to be 51 − 38 + 4 %, which agrees with the variability fractions provided by Buenzli et al., Radigan et al., and Metchev et al. for L4–L8 dwarfs. These spectral indices may support the future selection of the most likely variable directly imaged exoplanets for studies with the James Webb Space Telescope and as well as the 30 m telescopes.