TRANSITIONS IN THE CLOUD COMPOSITION OF HOT JUPITERS Parmentier, Vivien; Fortney, Jonathan J.; Showman, Adam P. ...
Astrophysical journal/The Astrophysical journal,
09/2016, Letnik:
828, Številka:
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ABSTRACT Over a large range of equilibrium temperatures, clouds shape the transmission spectrum of hot Jupiter atmospheres, yet their composition remains unknown. Recent observations show that the ...Kepler light curves of some hot Jupiters are asymmetric: for the hottest planets, the light curve peaks before secondary eclipse, whereas for planets cooler than ∼1900 K, it peaks after secondary eclipse. We use the thermal structure from 3D global circulation models to determine the expected cloud distribution and Kepler light curves of hot Jupiters. We demonstrate that the change from an optical light curve dominated by thermal emission to one dominated by scattering (reflection) naturally explains the observed trend from negative to positive offset. For the cool planets the presence of an asymmetry in the Kepler light curve is a telltale sign of the cloud composition, because each cloud species can produce an offset only over a narrow range of effective temperatures. By comparing our models and the observations, we show that the cloud composition of hot Jupiters likely varies with equilibrium temperature. We suggest that a transition occurs between silicate and manganese sulfide clouds at a temperature near 1600 K, analogous to the L/T transition on brown dwarfs. The cold trapping of cloud species below the photosphere naturally produces such a transition and predicts similar transitions for other condensates, including TiO. We predict that most hot Jupiters should have cloudy nightsides, that partial cloudiness should be common at the limb, and that the dayside hot spot should often be cloud-free.
Abstract
We present a new generation of substellar atmosphere and evolution models, appropriate for application to studies of L-, T-, and Y-type brown dwarfs and self-luminous extrasolar planets. The ...models describe the expected temperature-pressure profiles and emergent spectra of atmospheres in radiative-convective equilibrium with effective temperatures and gravities within the ranges 200 ≤
T
eff
≤ 2400 K and
2.5
≤
log
g
≤
5.5
. These ranges encompass masses from about 0.5 to 85 Jupiter masses for a set of metallicities (M/H = − 0.5 to + 0.5), C/O ratios (from 0.5 to 1.5 times that of solar), and ages. These models expand the diversity of model atmospheres currently available, notably to cooler effective temperatures and greater ranges in C/O. Notable improvements from past such models include updated opacities and atmospheric chemistry. Here we describe our modeling approach and present our initial tranche of models for cloudless, chemical equilibrium atmospheres. We compare the modeled spectra, photometry, and evolution to various data sets.
Condensation clouds in substellar atmospheres have been widely inferred from spectra and photometric variability. Up until now, their horizontally averaged vertical distribution and mean particle ...size have been largely characterized using models, one of which is the eddy diffusion-sedimentation model from Ackerman and Marley that relies on a sedimentation efficiency parameter, fsed, to determine the vertical extent of clouds in the atmosphere. However, the physical processes controlling the vertical structure of clouds in substellar atmospheres are not well understood. In this work, we derive trends in fsed across a large range of eddy diffusivities (Kzz), gravities, material properties, and cloud formation pathways by fitting cloud distributions calculated by a more detailed cloud microphysics model. We find that fsed is dependent on Kzz, but not gravity, when Kzz is held constant. fsed is most sensitive to the nucleation rate of cloud particles, as determined by material properties like surface energy and molecular weight. High surface energy materials form fewer, larger cloud particles, leading to large fsed (>1), and vice versa for materials with low surface energy. For cloud formation via heterogeneous nucleation, fsed is sensitive to the condensation nuclei flux and radius, connecting cloud formation in substellar atmospheres to the objects' formation environments and other atmospheric aerosols. These insights could lead to improved cloud models that help us better understand substellar atmospheres. For example, we demonstrate that fsed could increase with increasing cloud base depth in an atmosphere, shedding light on the nature of the brown dwarf L/T transition.
Abstract
Upcoming James Webb Space Telescope observations will allow us to study exoplanet and brown dwarf atmospheres in great detail. The physical interpretation of these upcoming high ...signal-to-noise observations requires precise atmospheric models of exoplanets and brown dwarfs. While several 1D and 3D atmospheric models have been developed in the past three decades, these models have often relied on simplified assumptions like chemical equilibrium and are also often not open-source, which limits their usage and development by the wider community. We present a Python-based 1Dl atmospheric radiative-convective equilibrium (RCE) model. This model has heritage from the Fortran-based code, which has been widely used to model the atmospheres of solar system objects, brown dwarfs, and exoplanets. In short, the basic capability of the original model is to compute the atmospheric state of the object under RCE given its effective or internal temperature, gravity, and host-star properties (if relevant). In the new model, which has been included within the well-utilized code-base
PICASO
, we have added these original features as well as the new capability of self-consistently treating disequilibrium chemistry. This code is widely applicable to hydrogen-dominated atmospheres (e.g., brown dwarfs and giant planets).
Context. A new class of exoplanets has emerged: the ultra hot Jupiters, the hottest close-in gas giants. The majority of them have weaker-than-expected spectral features in the 1.1−1.7 μm bandpass ...probed by HST/WFC3 but stronger spectral features at longer wavelengths probed by Spitzer. This led previous authors to puzzling conclusions about the thermal structures and chemical abundances of these planets. Aims. We investigate how thermal dissociation, ionization, H− opacity, and clouds shape the thermal structures and spectral properties of ultra hot Jupiters. Methods. We use the SPARC/MITgcm to model the atmospheres of four ultra hot Jupiters and discuss more thoroughly the case of WASP-121b. We expand our findings to the whole population of ultra hot Jupiters through analytical quantification of the thermal dissociation and its influence on the strength of spectral features. Results. We predict that most molecules are thermally dissociated and alkalies are ionized in the dayside photospheres of ultra hot Jupiters. This includes H2O, TiO, VO, and H2 but not CO, which has a stronger molecular bond. The vertical molecular gradient created by the dissociation significantly weakens the spectral features from H2O while the 4.5 μm CO feature remains unchanged. The water band in the HST/WFC3 bandpass is further weakened by the continuous opacity of the H− ions. Molecules are expected to recombine before reaching the limb, leading to order of magnitude variations of the chemical composition and cloud coverage between the limb and the dayside. Conclusions. Molecular dissociation provides a qualitative understanding of the lack of strong spectral features of water in the 1−2 μm bandpass observed in most ultra hot Jupiters. Quantitatively, our model does not provide a satisfactory match to the WASP-121b emission spectrum. Together with WASP-33b and Kepler-33Ab, they seem the outliers among the population of ultra hot Jupiters, in need of a more thorough understanding.
We address disequilibrium abundances of some simple molecules in the atmospheres of solar composition brown dwarfs and self-luminous extrasolar giant planets using a kinetics-based one-dimensional ...atmospheric chemistry model. Our approach is to use the full kinetics model to survey the parameter space with effective temperatures between 500 K and 1100 K. In all of these worlds, equilibrium chemistry favors CH sub(4) over CO in the parts of the atmosphere that can be seen from Earth, but in most disequilibrium favors CO. The small surface gravity of a planet strongly discriminates against CH sub(4) when compared to an otherwise comparable brown dwarf. If vertical mixing is like Jupiter's, the transition from methane to CO occurs at 500 K in a planet. Sluggish vertical mixing can raise this to 600 K, but clouds or more vigorous vertical mixing could lower this to 400 K. The comparable thresholds in brown dwarfs are 1100 + or - 100 K. Ammonia is also sensitive to gravity, but, unlike CH sub(4)/CO, the NH sub(3)/N sub(2) ratio is insensitive to mixing, which makes NH sub(3) a potential proxy for gravity. HCN may become interesting in high-gravity brown dwarfs with very strong vertical mixing. Detailed analysis of the CO-CH sub(4) reaction network reveals that the bottleneck to CO hydrogenation goes through methanol, in partial agreement with previous work. Simple, easy to use quenching relations are derived by fitting to the complete chemistry of the full ensemble of models. These relations are valid for determining CO, CH sub(4), NH sub(3), HCN, and CO sub(2) abundances in the range of self-luminous worlds we have studied, but may not apply if atmospheres are strongly heated at high altitudes by processes not considered here (e.g., wave breaking).
Opacities and chemical abundance data are crucial ingredients of ultracool dwarf and extrasolar giant planet atmosphere models. We report here on the detailed sources of molecular opacity data ...employed by our group for this application. We also present tables of Rosseland and Planck mean opacities, which are of use in some studies of the atmospheres, interiors, and evolution of planets and brown dwarfs. For the tables presented here we have included the opacities of important atomic and molecular species, including the alkali elements, pressure-induced absorption by hydrogen, and other significant opacity sources, but we neglect opacity from condensates. We report for each species how we have assembled molecular line data from a combination of public databases, laboratory data that is not yet in the public databases, and our own numerical calculations. We combine these opacities with abundances computed from a chemical equilibrium model using recently revised solar abundances to compute mean opacities. The chemical equilibrium calculation accounts for the settling of condensates in a gravitational field and is applicable to ultracool dwarf and extrasolar planetary atmospheres, but not circumstellar disks. We find that the inclusion of alkali atomic opacity substantially increases the mean opacities over those currently in the literature at densities relevant to the atmospheres and interiors of giant planets and brown dwarfs. We provide our opacity tables for public use and discuss their limitations.
Space-based high-contrast imaging mission concepts for studying rocky exoplanets in reflected light are currently under community study. We develop an inverse modeling framework to estimate the ...science return of such missions given different instrument design considerations. By combining an exoplanet albedo model, instrument noise model, and ensemble Markov chain Monte Carlo sampler, we explore retrievals of atmospheric and planetary properties for Earth twins as a function of signal-to-noise ratio (S/N) and resolution (R). Our forward model includes Rayleigh-scattering, single-layer water clouds with patchy coverage, and pressure-dependent absorption due to water vapor, oxygen, and ozone. We simulate data at R = 70 and 140 from 0.4 to 1.0 m with S/N = 5, 10, 15, and 20 at 550 nm (i.e., for HabEx/LUVOIR-type instruments). At these same S/Ns, we simulate data for WFIRST paired with a starshade, which includes two photometric points between 0.48 and 0.6 m and R = 50 spectroscopy from 0.6 to 0.97 m. Given our noise model for WFIRST-type detectors, we find that weak detections of water vapor, ozone, and oxygen can be achieved with observations with at least R = 70/S/N = 15 or R = 140/S/N = 10 for improved detections. Meaningful constraints are only achieved with R = 140/S/N = 20 data. The WFIRST data offer limited diagnostic information, needing at least S/N = 20 to weakly detect gases. Most scenarios place limits on planetary radius but cannot constrain surface gravity and, thus, planetary mass.
ABSTRACT Interpreting the spectra of brown dwarfs is key to determining the fundamental physical and chemical processes occurring in their atmospheres. Powerful Bayesian atmospheric retrieval tools ...have recently been applied to both exoplanet and brown dwarf spectra to tease out the thermal structures and molecular abundances to understand those processes. In this manuscript we develop a significantly upgraded retrieval method and apply it to the SpeX spectral library data of two benchmark late T dwarfs, Gl 570D and HD 3651B, to establish the validity of our upgraded forward model parameterization and Bayesian estimator. Our retrieved metallicities, gravities, and effective temperatures are consistent with the metallicity and presumed ages of the systems. We add the carbon-to-oxygen ratio as a new dimension to benchmark systems and find good agreement between carbon-to-oxygen ratios derived in the brown dwarfs and the host stars. Furthermore, we have for the first time unambiguously determined the presence of ammonia in the low-resolution spectra of these two late T dwarfs. We also show that the retrieved results are not significantly impacted by the possible presence of clouds, though some quantities are significantly impacted by uncertainties in photometry. This investigation represents a watershed study in establishing the utility of atmospheric retrieval approaches on brown dwarf spectra.
We observed two full orbital phase curves of the transiting brown dwarf KELT-1b, at 3.6 and 4.5 m, using the Spitzer Space Telescope. Combined with previous eclipse data from Beatty et al., we ...strongly detect KELT-1b's phase variation as a single sinusoid in both bands, with amplitudes of 964 36 ppm at 3.6 m and 979 54 ppm at 4.5 m, and confirm the secondary eclipse depths measured by Beatty et al. We also measure noticeable eastward hotspot offsets of 28 4 3 5 at 3.6 m and 18 6 5 2 at 4.5 m. Both the day-night temperature contrasts and the hotspot offsets we measure are in line with the trends seen in hot Jupiters, though we disagree with the recent suggestion of an offset trend by Zhang et al. Using an ensemble analysis of Spitzer phase curves, we argue that nightside clouds are playing a noticeable role in modulating the thermal emission from these objects, based on: (1) the lack of a clear trend in phase offsets with equilibrium temperature, (2) the sharp day-night transitions required to have non-negative intensity maps, which also resolves the inversion issues raised by Keating & Cowan, (3) the fact that all the nightsides of these objects appear to be at roughly the same temperature of 1000 K, while the dayside temperatures increase linearly with equilibrium temperature, and (4) the trajectories of these objects on a Spitzer color-magnitude diagram, which suggest colors only explainable via nightside clouds.
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