ABSTRACT The composition of a planet's atmosphere is determined by its formation, evolution, and present-day insolation. A planet's spectrum therefore may hold clues on its origins. We present a ..."chain" of models, linking the formation of a planet to its observable present-day spectrum. The chain links include (1) the planet's formation and migration, (2) its long-term thermodynamic evolution, (3) a variety of disk chemistry models, (4) a non-gray atmospheric model, and (5) a radiometric model to obtain simulated spectroscopic observations with James Webb Space Telescope and ARIEL. In our standard chemistry model the inner disk is depleted in refractory carbon as in the Solar System and in white dwarfs polluted by extrasolar planetesimals. Our main findings are: (1) envelope enrichment by planetesimal impacts during formation dominates the final planetary atmospheric composition of hot Jupiters. We investigate two, under this finding, prototypical formation pathways: a formation inside or outside the water iceline, called "dry" and "wet" planets, respectively. (2) Both the "dry" and "wet" planets are oxygen-rich (C/O < 1) due to the oxygen-rich nature of the solid building blocks. The "dry" planet's C/O ratio is <0.2 for standard carbon depletion, while the "wet" planet has typical C/O values between 0.1 and 0.5 depending mainly on the clathrate formation efficiency. Only non-standard disk chemistries without carbon depletion lead to carbon-rich C/O ratios >1 for the "dry" planet. (3) While we consistently find C/O ratios <1, they still vary significantly. To link a formation history to a specific C/O, a better understanding of the disk chemistry is thus needed.
Context. The cross-correlation technique is a well-tested method for exoplanet characterization, having lead to the detection of various molecules, to constraints on atmospheric temperature profiles, ...wind speeds, and planetary spin rates. A new, potentially powerful application of this technique is the measurement of atmospheric isotope ratios. In particular D/H can give unique insights into the formation and evolution of planets, and their atmospheres. Aims. In this paper we aim to study the detectability of molecular isotopologues in the high-dispersion spectra of exoplanet atmospheres, to identify the optimal wavelength ranges to conduct such studies, and to predict the required observational efforts – both with current and future ground-based instrumentation. Methods. High-dispersion (R = 100 000) thermal emission spectra, and in some cases reflection spectra, were simulated by self-consistent modeling of the atmospheric structures and abundances of exoplanets over a wide range of effective temperatures. These were synthetically observed with a telescope equivalent to the VLT and/or ELT, and analyzed using the cross-correlation technique, resulting in signal-to-noise ratio predictions for the 13CO, HDO, and CH3D isotopologues. Results. We find that for the best observable exoplanets, 13CO is well in range of current telescopes. We predict it will be most favorably detectable at 2.4 μm, just longward of the wavelength regions probed by several high-dispersion spectroscopic observations presented in the literature. CH3D can be best targeted at 4.7 μm, and may be detectable using 40 m-class telescopes for planets below 600 K in equilibrium temperature. In this case, the sky background becomes the dominating noise source for self-luminous planets. HDO is best targeted at 3.7 μm, and is less affected by sky background noise. 40 m-class telescopes may lead to its detection for planets with Tequ below 900 K. It could already be in the range of current 8 m-class telescopes in the case of quenched methane abundances. Finally, if Proxima Cen b is water-rich, the HDO isotopologue could be detected with the ELT in ~1 night of observing time in its reflected-light spectrum. Conclusions. Isotopologues will soon be a part of the exoplanet characterisation tools. Measuring D/H in exoplanets, and ratios of other isotopes, could become a prime science case for the first-light instrument METIS on the European ELT, especially for nearby temperate rocky and ice giant planets. This can provide unique insights in their history of icy-body enrichment and atmospheric evaporation processes.
ABSTRACT Many parameters constraining the spectral appearance of exoplanets are still poorly understood. We therefore study the properties of irradiated exoplanet atmospheres over a wide parameter ...range including metallicity, C/O ratio, and host spectral type. We calculate a grid of 1D radiative-convective atmospheres and emission spectra. We perform the calculations with our new Pressure-Temperature Iterator and Spectral Emission Calculator for Planetary Atmospheres (PETIT) code, assuming chemical equilibrium. The atmospheric structures and spectra are made available online. We find that atmospheres of planets with C/O ratios ∼1 and 1500 K can exhibit inversions due to heating by the alkalis because the main coolants CH4, H2O, and HCN are depleted. Therefore, temperature inversions possibly occur without the presence of additional absorbers like TiO and VO. At low temperatures we find that the pressure level of the photosphere strongly influences whether the atmospheric opacity is dominated by either water (for low C/O) or methane (for high C/O), or both (regardless of the C/O). For hot, carbon-rich objects this pressure level governs whether the atmosphere is dominated by methane or HCN. Further we find that host stars of late spectral type lead to planetary atmospheres which have shallower, more isothermal temperature profiles. In agreement with prior work we find that for planets with K the transition between water or methane dominated spectra occurs at C/O ∼ 0.7, instead of ∼1, because condensation preferentially removes oxygen.
Abstract
Ultrahot Jupiters are gas giants that orbit so close to their host star that they are tidally locked, causing a permanent hot dayside and a cooler nightside. Signatures of their nonuniform ...atmospheres can be observed with high-resolution transit transmission spectroscopy by resolving time-dependent velocity shifts as the planet rotates and varying areas of the evening and morning terminator are probed. These asymmetric shifts were seen for the first time in iron absorption in WASP-76b. Here, we search for other atoms/ions in the planets transmission spectrum and study the asymmetries in their signals. We detect Li
i
, Na
i
, Mg
i
, Ca
ii
, V
i
, Cr
i
, Mn
i
, Fe
i
, Ni
i
, and Sr
ii
, and tentatively detect H
i
, K
i
, and Co
i
, of which V, Cr, Ni, Sr
ii
, and Co have not been reported before. We notably do not detect Ti or Al, even though these species should be readily observable, and hypothesize this could be due to condensation or cold trapping. We find that the observed signal asymmetries in the detected species can be explained in different ways. We find a relation between the expected condensation or ionization temperatures and the strength of the observed asymmetry, which could indicate rain-out or recombination on the nightside. However, we also find a dependence on the signal broadening, which could imply a two-zoned atmospheric model, in which the lower atmosphere is dominated by a day-to-night wind, while the upper atmosphere is dominated by a vertical wind or outflow. These observations provide a new level of modeling constraint and will aid our understanding of atmospheric dynamics in highly irradiated planets.
Context.
Clouds are ubiquitous in exoplanet atmospheres and they represent a challenge for the model interpretation of their spectra. When generating a large number of model spectra, complex cloud ...models often prove too costly numerically, whereas more efficient models may be overly simplified.
Aims.
We aim to constrain the atmospheric properties of the directly imaged planet HR 8799e with a free retrieval approach.
Methods.
We used our radiative transfer code petitRADTRANS for generating the spectra, which we coupled to the PyMultiNest tool. We added the effect of multiple scattering which is important for treating clouds. Two cloud model parameterizations are tested: the first incorporates the mixing and settling of condensates, the second simply parameterizes the functional form of the opacity.
Results.
In mock retrievals, using an inadequate cloud model may result in atmospheres that are more isothermal and less cloudy than the input. Applying our framework on observations of HR 8799e made with the GPI, SPHERE, and GRAVITY, we find a cloudy atmosphere governed by disequilibrium chemistry, confirming previous analyses. We retrieve that C/O = 0.60
−0.08
+0.07
. Other models have not yet produced a well constrained C/O value for this planet. The retrieved C/O values of both cloud models are consistent, while leading to different atmospheric structures: either cloudy or more isothermal and less cloudy. Fitting the observations with the self-consistent Exo-REM model leads to comparable results, without constraining C/O.
Conclusions.
With data from the most sensitive instruments, retrieval analyses of directly imaged planets are possible. The inferred C/O ratio of HR 8799e is independent of the cloud model and thus appears to be a robust. This C/O is consistent with stellar, which could indicate that the HR 8799e formed outside the CO
2
or CO iceline. As it is the innermost planet of the system, this constraint could apply to all HR 8799 planets.
Context.
The anomalously large radii of hot Jupiters are still not fully understood, and all of the proposed explanations are based on the idea that these close-in giant planets possess hot ...interiors. Most of the mechanisms proposed have been tested on a handful of exoplanets.
Aims.
We approach the radius anomaly problem by adopting a statistical approach. We want to infer the internal luminosity for the sample of hot Jupiters, study its effect on the interior structure, and put constraints on which mechanism is the dominant one.
Methods.
We developed a flexible and robust hierarchical Bayesian model that couples the interior structure of exoplanets to the observed properties of close-in giant planets. We applied the model to 314 hot Jupiters and inferred the internal luminosity distribution for each planet and studied at the population level (i) the mass–luminosity–radius distribution and as a function of equilibrium temperature the distributions of the (ii) heating efficiency, (iii) internal temperature, and the (iv) pressure of the radiative–convective–boundary (RCB).
Results.
We find that hot Jupiters tend to have high internal luminosity with 10
4
L
J
for the largest planets. As a result, we show that all the inflated planets have hot interiors with an internal temperature ranging from 200 up to 800 K for the most irradiated ones. This has important consequences on the cooling rate and we find that the RCB is located at low pressures between 3 and 100 bar. Assuming that the ultimate source of the extra heating is the irradiation from the host star, we also illustrate that the heating efficiency increases with increasing equilibrium temperature and reaches a maximum of 2.5% at ~1860 K, beyond which the efficiency decreases, which is in agreement with previous results. We discuss our findings in the context of the proposed heating mechanisms and illustrate that ohmic dissipation, the advection of potential temperature, and thermal tides are in agreement with certain trends inferred from our analysis and thus all three models can explain various aspects of the observations.
Conclusions.
We provide new insights on the interior structure of hot Jupiters and show that with our current knowledge, it is still challenging to firmly identify the universal mechanism driving the inflated radii.
KELT-9 b, the hottest known exoplanet, with
T
eq
~ 4400 K, is the archetype of a new planet class known as ultra-hot Jupiters. These exoplanets are presumed to have an atmosphere dominated by neutral ...and ionized atomic species. In particular, H
α
and H
β
Balmer lines have been detected in the KELT-9 b upper atmosphere, suggesting that hydrogen is filling the planetary Roche lobe and escaping from the planet. In this work, we detected
δ
Scuti-type stellar pulsation (with a period
P
puls
= 7.54 ± 0.12 h) and studied the Rossiter-McLaughlin effect (finding a spin-orbit angle
λ
= −85.01° ± 0.23°) prior to focussing on the Balmer lines (H
α
to H
ζ
) in the optical transmission spectrum of KELT-9 b. Our HARPS-N data show significant absorption for H
α
to H
δ
. The precise line shapes of the H
α
, H
β
, and H
γ
absorptions allow us to put constraints on the thermospheric temperature. Moreover, the mass loss rate, and the excited hydrogen population of KELT-9 b are also constrained, thanks to a retrieval analysis performed with a new atmospheric model. We retrieved a thermospheric temperature of
T
= 13 200
−720
+800
K and a mass loss rate of
Ṁ
= 10
12.8±0.3
g s
−1
when the atmosphere was assumed to be in hydrodynamical expansion and in local thermodynamic equilibrium (LTE). Since the thermospheres of hot Jupiters are not expected to be in LTE, we explored atmospheric structures with non-Boltzmann equilibrium for the population of the excited hydrogen. We do not find strong statistical evidence in favor of a departure from LTE. However, our non-LTE scenario suggests that a departure from the Boltzmann equilibrium may not be sufficient to explain the retrieved low number densities of the excited hydrogen. In non-LTE, Saha equilibrium departure via photo-ionization, is also likely to be necessary to explain the data.
Ultra-hot Jupiters are emerging as a new class of exoplanets. Studying their chemical compositions and temperature structures will improve our understanding of their mass loss rate as well as their ...formation and evolution. We present the detection of ionized calcium in the two hottest giant exoplanets – KELT-9b and WASP-33b. By using transit datasets from CARMENES and HARPS-N observations, we achieved high-confidence-level detections of Ca II using the cross-correlation method. We further obtain the transmission spectra around the individual lines of the Ca II H&K doublet and the near-infrared triplet, and measure their line profiles. The Ca II H&K lines have an average line depth of 2.02 ± 0.17% (effective radius of 1.56 Rp) for WASP-33b and an average line depth of 0.78 ± 0.04% (effective radius of 1.47 Rp) for KELT-9b, which indicates that the absorptions are from very high upper-atmosphere layers close to the planetary Roche lobes. The observed Ca II lines are significantly deeper than the predicted values from the hydrostatic models. Such a discrepancy is probably a result of hydrodynamic outflow that transports a significant amount of Ca II into the upper atmosphere. The prominent Ca II detection with the lack of significant Ca I detection implies that calcium is mostly ionized in the upper atmospheres of the two planets.
We present broad-band photometry of five transits in the planetary system WASP-36, totalling 17 high-precision light curves. Four of the transits were simultaneously observed in four passbands (g′, ...r′, i′, z′), using the telescope-defocusing technique, and achieving scatters of less than 1 mmag per observation. We used these data to improve the measured orbital and physical properties of the system, and obtain an optical transmission spectrum of the planet. We measured a decreasing radius from bluer to redder passbands with a confidence level of more than 5σ. The radius variation is roughly 11 pressure scale heights between the g′ and the z′ bands. This is too strong to be Rayleigh scattering in the planetary atmosphere, and implies the presence of a species which absorbs strongly at bluer wavelengths.
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