Grid Of Model Atmospheres Research Articles

Physical Parameters and Properties of 20 Cold Brown Dwarfs in JWST

We present a comprehensive analysis of 20 T and Y dwarfs using spectroscopy from the Near-Infrared Spectrograph (NIRSpec) CLEAR/PRISM and Mid-Infrared Instrument (MIRI) low-resolution spectrometer instruments on the James Webb Space Telescope. To characterize the atmospheric parameters, we utilize two atmospheric model grids: the Sonora Elf Owl and ATMO2020++. The effective temperatures derived from the two models are relatively consistent, and metallicities are both close to solar values. However, significant discrepancies are found in other parameters, particularly in surface gravity, with the values obtained from the Sonora Elf Owl models typically being about 1 dex lower than those from the ATMO2020++ models. Further comparisons using the ATMO2020 models highlight that the adiabatic convective process introduced in the ATMO2020++ models has a significant impact on the determination of surface gravity. Using the fitted effective temperatures and absolute parallaxes from the literature, we derive radii for the brown dwarfs, which range from approximately 0.8–1.2 R Jup. The estimated masses and ages, derived using evolutionary tracks, indicate that most brown dwarfs in our sample have masses below 30 M Jup and are younger than 6 Gyr. Specifically, Y dwarfs have masses ranging from 2 to 20 M Jup and ages between 0.1 and 6.7 Gyr. In addition, we discuss the determination of atmospheric parameters using only NIRSpec or MIRI spectra. Comparisons with results from the combined spectra show that the effective temperatures and surface gravities derived solely from NIRSpec spectra are largely consistent with those obtained from the combined spectra.

The Sonora Substellar Atmosphere Models. III. Diamondback: Atmospheric Properties, Spectra, and Evolution for Warm Cloudy Substellar Objects

We present a new grid of cloudy atmosphere and evolution models for substellar objects. These models include the effect of refractory cloud species, including silicate clouds, on the spectra and evolution. We include effective temperatures from 900 to 2400 K and surface gravities from log g = 3.5 to 5.5, appropriate for a broad range of objects with masses between 1 and 84 M J. Model pressure–temperature structures are calculated assuming radiative–convective and chemical equilibrium. We consider the effect of both clouds and metallicity on the atmospheric structure, resulting spectra, and thermal evolution of substellar worlds. We parameterize clouds using the A. S. Ackerman & M. S. Marley cloud model, including cloud parameter f sed values from 1 to 8; we include three metallicities (−0.5, 0.0, and +0.5). Refractory clouds and metallicity both alter the evolution of substellar objects, changing the inferred temperature at a given age by up to 100–200 K. For solar-metallicity evolution models including clouds in warm objects, we find a hydrogen-burning minimum mass of 70.2 M J, close to empirical measurements; we find a deuterium-burning minimum mass of 12.05 M J (50% of initial D burned). We compare to the observed photometry of brown dwarfs, finding broad agreement with the measured photometry. We publish the spectra, evolution, and other data products online with open access on Zenodo (doi:10.5281/zenodo.12735103).

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 James Webb Space Telescope Guaranteed Time Observer program to obtain moderate resolution spectra (R ∼ 2700, 3–5 μm) of a high-contrast companion with the NIRSpec integral field unit (IFU). HD 19467 B is an old, ∼9 Gyr, companion to a solar-type star with multiple measured dynamical masses. The spectra show detections of CO, CO2, CH4, and H2O. 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 nonequilibrium 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 to 1200 K, a surface gravity of 4.50 dex, with a range of 4.14–5.00, and deep vertical mixing, log10(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 ∼ 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 71.6−4.6+5.3MJup , in agreement with the mass range we derive from evolutionary models, which we find to be 63–75 M 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 ∼1.″6 with a contrast of 10−4 in the 3–5 μm range.

An extended and refined grid of 3D STAGGER model atmospheres

An extended and refined grid of 3D STAGGER model atmospheres

Revisiting the statistical equilibrium of H- in stellar atmospheres

The negative hydrogen ion is, almost without exception, treated in local thermodynamic equilibrium (LTE) in the modelling of F, G, and K stars, where it is the dominant opacity source in the visual spectral region. This assumption rests in practice on a study from the 1960s. Since that work, knowledge of relevant atomic processes and theoretical calculations of stellar atmospheres and their spectra have advanced significantly, but this question has not been reexamined. We present calculations based on a slightly modified analytical model that includes H and together with modern atomic data and a grid of 1D LTE theoretical stellar atmosphere models with stellar parameters ranging from T$_ eff = 4000$ to 7000 K, $ g = 1$ to 5 cm/s$^2$, and Fe/H $=-3$ to 0. We find direct non-LTE effects on populations in spectrum-forming regions, continua, and spectral lines of about 1--2<!PCT!> in stars with higher T$_ eff $ and/or lower $ g$. Effects in models for solar parameters are smaller by a factor of 10, about 0.1--0.2<!PCT!>, and are practically absent in models with lower T$_ eff $ and/or higher $ g$. These departures from LTE found in our calculations originate from the radiative recombination of electrons with hydrogen to form exceeding photodetachment, that is, overrecombination. Modern atomic data are not a source of significant differences compared to the previous work, although detailed data for processes on resolved with vibrational and rotational states provide a more complete and complex picture of the role of in the equilibrium of In the context of modern studies of stellar spectra at the percent level, our results suggest that this question requires further attention, including a more extensive reaction network, and indirect effects due to non-LTE electron populations.

Young stellar objects from the LAMOST and ZTF surveys: Physical properties, classification, and light curve analysis

Context. The study of young stellar objects (YSOs) not only enhances our understanding of star formation and stellar evolution, but also contributes to broader areas of astrophysics, including planetary science, galactic dynamics, and astrochemistry. Aims. We aimed to comprehensively analyse 657 YSOs and provide their physical parameter measurements using data from Zwicky Transient Facility (ZTF) g- and r-band light curves and the Gaia, WISE, 2MASS, and LAMOST databases. Specifically, we sought to identify periodicity in the light curves and classify the YSOs based on the Q – M variability plane, which enabled us to quantify flux asymmetry and quasi-periodicity. Methods. To achieve our objectives, we conducted a meticulous examination of the light curves obtained from the ZTF and estimated the physical parameters of the YSOs. These parameters were discerned by integrating stellar model atmosphere grids, photometric data, Gaia DR3 parallaxes, and pre-main-sequence evolutionary tracks. We employed the Q – M variability plane to classify the YSOs and determine the presence of periodic patterns. Additionally, we analysed the distribution of variability slope angles in the colour-magnitude diagram (CMD) to discern patterns associated with extinction-driven and accretion-related variability. Results. Our analysis revealed significant findings regarding the variability patterns and physical characteristics of the YSOs. Among the 657 objects analysed, 37 exhibited periodic variability and 2 displayed multi-period behaviour. Furthermore, we identified distinct variability patterns, including quasi-periodic symmetry, quasi-periodic dipping, aperiodic dipping, bursting behaviour, stochastic variability, and long-timescale variations. Notably, the distribution of variability slope angles in the CMD varied between dippers and bursters, indicating different underlying variability drivers. Additionally, we observed that YSOs classified as classical T Tauri stars and weak-line T Tauri stars exhibited contrasting light curve characteristics, with Class II YSOs displaying asymmetry and Class III YSOs showing (quasi-)periodic variations. These findings underscore the importance of considering variability patterns when classifying and determining the nature of YSOs.

The Spectral ANalog of Dwarfs (SAND): New Model Atmospheres with Varying Chemistry for Galactic Archaeology with Ultracool Dwarfs

The atmospheres of Ultracool Dwarfs (UCDs) are dominated by molecular chemistry, which makes their spectra and photometry particularly sensitive to elemental abundances. With lifespans in excess of the age of the universe, UCDs serve as chemical tracers in every component of the Milky Way. In this study, we present the Spectral ANalog of Dwarfs (SAND) grid of low-temperature model atmospheres that span T eff from 700 to 4000 K, log(g) from 4.0 to 6.0, [Fe/H] from −2.4 to +0.3, and a range of [α/Fe] that matches the Galactic distribution inferred from earlier spectroscopic surveys. The SAND grid primarily aims to model the spectra of brown dwarfs in the halo and thick disk of the Milky Way, and metal-poor UCDs in globular clusters.

M3DIS - A grid of 3D radiation-hydrodynamics stellar atmosphere models for stellar surveys. I. Procedure, validation, and the Sun

Large-scale stellar surveys, such as SDSS-V, 4MOST, WEAVE, and PLATO, require accurate atmospheric models and synthetic spectra of stars for accurate analyses of fundamental stellar parameters and chemical abundances. The primary goal of our work is to develop a new approach to solve radiation-hydrodynamics (RHD) and generate model stellar spectra in a self-consistent and highly efficient framework. We build upon the Copenhagen legacy RHD code, the MULTI3D non-local thermodynamic equilibrium (NLTE) code, and the DISPATCH high-performance framework. The new approach allows us to calculate 3D RHD models of stellar atmospheres on timescales of a few thousand CPU hours and to perform subsequent spectrum synthesis in local thermodynamic equilibrium (LTE) or NLTE for the desired physical conditions within the parameter space of FGK-type stars. We compare the 3D RHD solar model with other available models and validate its performance against solar observations, including the centre-to-limb variation of intensities and key solar diagnostic lines of H and Fe. We show that the performance of the new code allows to overcome the main bottleneck in 3D NLTE spectroscopy and enables calculations of multi-dimensional grids of synthetic stellar observables for comparison with modern astronomical observations.

Entropy-calibrated stellar modeling: Testing and improving the use of prescriptions for the entropy of adiabatic convection

Modeling the convection process is a long-standing problem in stellar physics. To date, all ad hoc models have relied on a free parameter, $ (among others) that has no real physical justification and is therefore poorly constrained. However, a link exists between this free parameter and the entropy of the stellar adiabat. There are existing prescriptions, derived from 3D stellar atmospheric models, that treat entropy as a function of stellar atmospheric parameters (effective temperature, surface gravity, and chemical composition). This can offer sufficient constraints on alpha through the development of entropy-calibrated models. However, several questions have arisen as these models are increasingly used with respect to which prescription should be used and whether it ought to be used in its original form, along with the impacts of uncertainties on entropy-calibrated models. We aim to study the three existing prescriptions in detail and determine which of them demonstrate the most optimal performance and how it should be applied. We implemented the entropy-calibration method into the stellar evolution code ( and performed comparisons with the Sun and the system. In addition, we used data from the CIFIST grid of 3D atmosphere models to evaluate the accuracy of the prescriptions. Of the three entropy prescriptions currently available, we determined the one that has the best functional form for reproducing the entropies of the 3D models. However, the coefficients involved in this formulation must not be taken from the original paper because they were calibrated against a flawed set of entropies. We also demonstrate that the entropy obtained from this prescription should be corrected for the evolving chemical composition and for an entropy offset different between various EoS tables. This must be done following a precise procedure to ensure that the classical parameters obtained from the models are not strongly biased. Finally, we provide a data table with entropy of the adiabat of the CIFIST grid, along with the fits for these entropies. Thanks to a precise examination of entropy-calibrated modeling, we are able to offer our recommendations with respect to which adiabatic entropy prescription to use, how to correct it, and how to implement the method into a stellar evolution code.

Direct-imaging Discovery of a Substellar Companion Orbiting the Accelerating Variable Star HIP 39017

We present the direct-imaging discovery of a substellar companion (a massive planet or low-mass brown dwarf) to the young, γ Doradus (γ Dor)-type variable star HIP 39017 (HD 65526). The companion’s SCExAO/CHARIS JHK (1.1–2.4 μm) spectrum and Keck/NIRC2 L′ photometry indicate that it is an L/T transition object. A comparison of the JHK+L ′ spectrum to several atmospheric model grids finds a significantly better fit to cloudy models than cloudless models. Orbit modeling with relative astrometry and precision stellar astrometry from Hipparcos and Gaia yields a semimajor axis of 23.8−6.1+8.7 au, a dynamical companion mass of 30−12+31 M J, and a mass ratio of ∼1.9%, properties most consistent with low-mass brown dwarfs. However, its mass estimated from luminosity models is a lower ∼13.8 M J due to an estimated young age (≲115 Myr); using a weighted posterior distribution informed by conservative mass constraints from luminosity evolutionary models yields a lower dynamical mass of 23.6−7.4+9.1 M J and a mass ratio of ∼1.4%. Analysis of the host star’s multifrequency γ Dor-type pulsations, astrometric monitoring of HIP 39017 b, and Gaia Data Release 4 astrometry of the star will clarify the system age and better constrain the mass and orbit of the companion. This discovery further reinforces the improved efficiency of targeted direct-imaging campaigns informed by long-baseline, precision stellar astrometry.

The Sonora Substellar Atmosphere Models. IV. Elf Owl: Atmospheric Mixing and Chemical Disequilibrium with Varying Metallicity and C/O Ratios

Disequilibrium chemistry due to vertical mixing in the atmospheres of many brown dwarfs and giant exoplanets is well established. Atmosphere models for these objects typically parameterize mixing with the highly uncertain K zz diffusion parameter. The role of mixing in altering the abundances of C-N-O-bearing molecules has mostly been explored for atmospheres with a solar composition. However, atmospheric metallicity and the C/O ratio also impact atmospheric chemistry. Therefore, we present the Sonora Elf Owl grid of self-consistent cloud-free 1D radiative-convective equilibrium model atmospheres for JWST observations, which includes a variation in K zz across several orders of magnitude and also encompasses subsolar to supersolar metallicities and C/O ratios. We find that the impact of K zz on the T(P) profile and spectra is a strong function of both T eff and metallicity. For metal-poor objects, K zz has large impacts on the atmosphere at significantly higher T eff than in metal-rich atmospheres, where the impact of K zz is seen to occur at lower T eff. We identify significant spectral degeneracies between varying K zz and metallicity in multiple wavelength windows, in particular, at 3–5 μm. We use the Sonora Elf Owl atmospheric grid to fit the observed spectra of a sample of nine early to late T-type objects from T eff = 550–1150 K. We find evidence for very inefficient vertical mixing in these objects, with inferred K zz values lying in the range between ∼101 and 104 cm2 s−1. Using self-consistent models, we find that this slow vertical mixing is due to the observations, which probe mixing in the deep detached radiative zone in these atmospheres.

Spectroscopic analysis of hot, massive stars in large spectroscopic surveys with de-idealized models

ABSTRACT Upcoming large-scale spectroscopic surveys with e.g. WEAVE (William herschel telescope Enhanced Area Velocity Explorer) and 4MOST (4-metre Multi-Object Spectroscopic Telescope) will provide thousands of spectra of massive stars, which need to be analysed in an efficient and homogeneous way. Usually, studies of massive stars are limited to samples of a few hundred objects, which pushes current spectroscopic analysis tools to their limits because visual inspection is necessary to verify the spectroscopic fit. Often uncertainties are only estimated rather than derived and prior information cannot be incorporated without a Bayesian approach. In addition, uncertainties of stellar atmospheres and radiative transfer codes are not considered as a result of simplified, inaccurate, or incomplete/missing physics or, in short, idealized physical models. Here, we address the question of ‘How to compare an idealized model of complex objects to real data?’ with an empirical Bayesian approach and maximum a posteriori approximations. We focus on application to large-scale optical spectroscopic studies of complex astrophysical objects like stars. More specifically, we test and verify our methodology on samples of OB stars in 30 Doradus region of the Large Magellanic Clouds using a grid of fastwind model atmospheres. Our spectroscopic model de-idealization analysis pipeline takes advantage of the statistics that large samples provide by determining the model error to account for the idealized stellar atmosphere models, which are included into the error budget. The pipeline performs well over a wide parameter space and derives robust stellar parameters with representative uncertainties.

Sulfur dioxide in the mid-infrared transmission spectrum of WASP-39b.

The recent inference of sulfur dioxide (SO2) in the atmosphere of the hot (approximately 1,100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations1-3 suggests that photochemistry is a key process in high-temperature exoplanet atmospheres4. This is because of the low (<1 ppb) abundance of SO2 under thermochemical equilibrium compared with that produced from the photochemistry of H2O and H2S (1-10 ppm)4-9. However, the SO2 inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 μm and, therefore, the detection of other SO2 absorption bands at different wavelengths is needed to better constrain the SO2 abundance. Here we report the detection of SO2 spectral features at 7.7 and 8.5 μm in the 5-12-μm transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS)10. Our observations suggest an abundance of SO2 of 0.5-25 ppm (1σ range), consistent with previous findings4. As well as SO2, we find broad water-vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 μm. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy-element content (metallicity) for WASP-39b of approximately 7.1-8.0 times solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.

Volatile-to-sulfur Ratios Can Recover a Gas Giant’s Accretion History

The newfound ability to detect SO2 in exoplanet atmospheres presents an opportunity to measure sulfur abundances and so directly test between competing modes of planet formation. In contrast to carbon and oxygen, whose dominant molecules are frequently observed, sulfur is much less volatile and resides almost exclusively in solid form in protoplanetary disks. This dichotomy leads different models of planet formation to predict different compositions of gas giant planets. Whereas planetesimal-based models predict roughly stellar C/S and O/S ratios, pebble-accretion models more often predict superstellar ratios. To explore the detectability of SO2 in transmission spectra and its ability to diagnose planet formation, we present a grid of atmospheric photochemical models and corresponding synthetic spectra for WASP-39b (where SO2 has been detected). Our 3D grid contains 113 models (spanning 1–100× the solar abundance ratio of C, O, and S) for thermal profiles corresponding to the morning and evening terminators, as well as mean terminator transmission spectra. Our models show that for a WASP-39b-like O/H and C/H enhancement of ∼10× solar, SO2 can only be seen for C/S and O/S ≲ 1.5× solar, and that WASP-39b’s reported SO2 abundance of 1–10 ppm may be more consistent with planetesimal accretion than with pebble-accretion models (although some pebble models also manage to predict similarly low ratios). More extreme C/S and O/S ratios may be detectable in higher-metallicity atmospheres, suggesting that smaller and more metal-rich gas and ice giants may be particularly interesting targets for testing planet formation models. Future studies should explore the dependence of SO2 on a wider array of planetary and stellar parameters, both for the prototypical SO2 planet WASP-39b, as well as for other hot Jupiters and smaller gas giants.

The First JWST Spectral Energy Distribution of a Y Dwarf

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/Δλ ∼100) spectroscopy from 1–12 μm and three photometric points at 15, 18, and 21 μm. The spectrum exhibits numerous fundamental, overtone, and combination rotational–vibrational bands of H2O, CH4, CO, CO2, and NH3, including the previously unidentified ν 3 band of NH3 at 3 μm. Using a Rayleigh–Jeans tail to account for the flux emerging at wavelengths greater than 21 μm, we measure a bolometric luminosity of 1.523 ± 0.090 × 1020 W. We determine a semiempirical effective temperature estimate of 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.

Modeling Land‐Atmosphere Coupling at Cloud‐Resolving Scale Within the Multiple Atmosphere Multiple Land (MAML) Framework in SP‐E3SM

AbstractRepresenting subgrid variabilities of land surface processes and their upscaled effects is crucial for global climate modeling. Here, we implement a multiple atmosphere multiple land (MAML) framework in the superparamaterized version of E3SM (SP‐E3SM) to explicitly simulate the subgrid variabilities of land states and fluxes at cloud‐resolving scale and their interactions with atmosphere. Comparing to the standard SP‐E3SM in which all the atmospheric columns of the cloud resolving model embedded within the global atmospheric model grid interact with the same land surface (i.e., multiple atmosphere single land (MASL)), the impact of MAML on the strength of land‐atmosphere coupling is limited, partly because the current implementation mainly facilitates one‐way coupling between the cloud‐resolving model and the land surface model. Despite such limitation, MAML increases the surface latent heat flux at the expense of sensible heat flux, and increases precipitation in India, Amazon, and Central Africa, reducing the model dry bias compared to the standard SP‐E3SM. By employing a normalized gross moist stability (NGMS) diagnostic framework, we find that the increase in precipitation minus evaporation (P‐E) is primarily driven by the change in large‐scale moisture convergence, particularly by the increase of water vapor in the lower atmosphere, while the local effect of total surface energy flux plays a minor role in the P‐E change. More specifically, MAML changes the surface energy partitioning (evaporative fraction), increases the atmosphere water vapor, and further increases P‐E by decreasing the NGMS. Finally, future development in the MAML framework is discussed.

A grid of Non-LTE line-blanketed atmosphere structures and synthetic spectra for subdwarfs

We present an update of the grid of detailed atmosphere models and homogeneous synthetic spectra for hot, high-gravity subdwarf stars. High-resolution spectra and synthetic photometry were calculated in the wavelength range 1,000 Å – 10,000 Å using Non-LTE extensively line-blanketed atmosphere structures.

First Release of PLATO Consortium Stellar Limb-darkening Coefficients

We release the first grid of stellar limb-darkening coefficients (LDCs) and intensity profiles (IPs) computed by the consortium of the PLAnetary Transits and Oscillations of stars (PLATO), the next medium-class (M3) mission under development by the European Space Agency to be launched in 2026. We have performed spectral synthesis with TurboSpectrum on a grid of MARCS model atmospheres. Finally, we adopted ExoTETHyS to convolve the high-resolution spectra (R = 2 × 105) with the state-of-the-art response functions for all the PLATO cameras, and computed the LDCs that best approximate the convolved IPs. In addition to the PLATO products, we provide new LDCs and IPs for the Kepler mission, based on the same grid of stellar atmospheric models and calculation procedures. The data can be downloaded from the following link: https://doi.org/10.5281/zenodo.7339706.

Probing the Extent of Vertical Mixing in Brown Dwarf Atmospheres with Disequilibrium Chemistry

Evidence of disequilibrium chemistry due to vertical mixing in the atmospheres of many T- and Y-dwarfs has been inferred due to enhanced mixing ratios of CO and reduced NH3. Atmospheric models of planets and brown dwarfs typically parameterize this vertical mixing phenomenon with the vertical eddy diffusion coefficient, K zz . While K zz can perhaps be approximated in the convective regions in the atmosphere with mixing length theory, in radiative regions, the strength of vertical mixing is uncertain by many orders of magnitude. With a new grid of self-consistent 1D model atmospheres from T eff of 400–1000 K, computed with a new radiative-convective equilibrium python code PICASO 3.0, we aim to assess how molecular abundances and corresponding spectra can be used as a probe of depth-dependent K zz . At a given surface gravity, we find nonmonotonic behavior in the CO abundance as a function of T eff, as chemical abundances are sometimes quenched in either of two potential atmospheric convective zones, or quenched in either of two possible radiative zones. The temperature structure and chemical quenching behavior also change with gravity. We compare our models with available near-infrared and M-band spectroscopy of several T- and Y-dwarfs and assess their atmospheric vertical mixing profiles. We also compare to color–magnitude diagrams and make predictions for James Webb Space Telescope spectra. This work yields new constraints, and points the way to significant future gains, in determining K zz , a fundamental atmospheric parameter in substellar atmospheres, with significant implications for chemistry and cloud modeling.

Moderate-resolution K-band Spectroscopy of the Substellar Companion VHS 1256 b

We present moderate-resolution (R ∼ 4000) K-band spectra of the planetary-mass companion VHS 1256 b. The data were taken with the OSIRIS integral field spectrograph at the W.M. Keck Observatory. The spectra reveal resolved molecular lines from H2O and CO. The spectra are compared to custom PHOENIX atmosphere model grids appropriate for young, substellar objects. We fit the data using a Markov chain Monte Carlo forward-modeling method. Using a combination of our moderate-resolution spectrum and low-resolution broadband data from the literature, we derive an effective temperature of 1240 K, with a range of 1200–1300 K, a surface gravity of log g = 3.25, with a range of 3.25–3.75, and a cloud parameter of , with a range of 6.0–6.6. These values are consistent with previous studies, regardless of the new, larger system distance from GAIA EDR3 (21.15 0.22 pc). We derive a C/O ratio of for VHS 1256b. Both our OSIRIS data and spectra from the literature are best modeled when using a larger 3 μm grain size for the clouds than used for hotter objects, consistent with other sources in the L/T transition region. VHS 1256 b offers an opportunity to look for systematics in the modeling process that may lead to the incorrect derivation of properties like C/O ratio in the high contrast regime.