arXiv subjects

Aaron David Schneider

Publications and source records attributed to Aaron David Schneider.

13 recordsLinked to original sources

arXiv2024

A formation pathway for terrestrial planets with moderate water content involving atmospheric-volatile recycling

Of the many recently discovered terrestrial exoplanets, some are expected to harbor moderate water mass fractions of a few percent. The formation pathways that can produce planets with these water mass fractions are not fully understood. Here, we use the code chemcomp, which consists of a semi-analytical 1D protoplanetary disk model harboring a migrating and accreting planet, to model the growth and composition of planets with moderate water mass fractions by pebble accretion in a protoplanetary disk around a TRAPPIST-1 analog star. This star is accompanied by seven terrestrial planets, of which the outer four planets likely contain water mass fractions of between 1\% and 10\%. We adopt a published model that considers the evaporation of pebbles in the planetary envelope, from where recycling flows can transport the volatile vapor back into the disk. We find that with this model, the planetary water content depends on the influx rate of pebbles onto the planet. A decreasing pebble influx with time reduces the envelope temperature and consequently allows the formation of planets with moderate water mass fractions as inferred for the outer TRAPPIST-1 planets for a number of different simulation configurations. This is further evidence that the recycling of vapor is an important component of planet formation needed to explain the vast and diverse population of exoplanets.

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arXiv2024

Examining the detectability of ringing on highly eccentric exoplanets

Eccentric exoplanets offer an opportunity to study the response of an atmosphere to changing thermal forcing and the robustness of the super-rotating equatorial jet seen on tidally locked hot Jupiters. However, the atmospheric dynamics on eccentric planets strongly depend on the planetary rotation period, which is difficult to constrain observationally. The ringing phenomenon, whereby the observed emission increases and decreases after the periastron passage as the flash-heated hemisphere rotates into and out of view, can provide a tight constraint on rotation. We studied five highly eccentric transiting exoplanets HAT-P-2 b, HD 80606 b, TOI-3362 b, TOI-4127 b and HD 17156 b to find which displays strong ringing signals that are sufficiently strong for the James Webb Space Telescope (JWST) to detect. We implemented the treatment of eccentricity and non-synchronous rotation in the non-grey climate model expeRT/MITgcm and generated synthetic light curves. We find four detectable ringing peaks on HD 80606 b and some undetectable ringing on TOI-4127 b and HD 17156 b. The lack of clouds, photo-chemistry and obliquity in our models may have led us to overestimate the amplitude of the ringing however. The strength of the ringing signal is mostly determined by the eccentricity, planetary rotation period, planet-to-star radius ratio and apparent magnitude of the system. We searched for more exoplanets that could show ringing but found no candidates as promising as HD 80606 b. We recommend prioritising HD 80606 b as a target for ringing with JWST. A baseline of five days after the periastron passage would capture three ringing peaks, which is sufficient to tightly constrain the planetary rotation period. An extension to seven days would add a fourth peak, which would allow us to verify the rotation period.

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arXiv2024

Photodissociation and induced chemical asymmetries on ultra-hot gas giants. A case study of HCN on WASP-76 b

Recent observations have resulted in the detection of chemical gradients on ultra-hot gas giants. Notwithstanding their high temperature, chemical reactions in ultra-hot atmospheres may occur in disequilibrium, due to vigorous day-night circulation and intense UV radiation from their stellar hosts. The goal of this work is to explore whether photochemistry is affecting the composition of ultra-hot giant planets, and if it can introduce horizontal chemical gradients. In particular, we focus on hydrogen cyanide (HCN) on WASP-76 b, as it is a photochemically active molecule with a reported detection on only one side of this planet. We use a pseudo-2D chemical kinetics code to model the chemical composition of WASP-76 b along its equator. Our approach improves on chemical equilibrium models by computing vertical mixing, horizontal advection, and photochemistry. We find that production of HCN is initiated through thermal and photochemical dissociation of CO and N$_2$ on the day side of WASP-76 b. The resulting radicals are subsequently transported to the night side via the equatorial jet stream, where they recombine into different molecules. This process results in an HCN gradient with a maximal abundance on the planet's morning limb. We verified that photochemical dissociation is a necessary condition for this mechanism, as thermal dissociation alone proves insufficient. Other species produced via night-side disequilibrium chemistry are SO$_2$ and S$_2$. Our model acts as a proof of concept for chemical gradients on ultra-hot exoplanets. We demonstrate that even ultra-hot planets can exhibit disequilibrium chemistry and recommend that future studies do not neglect photochemistry in their analyses of ultra-hot planets.

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arXiv2023

Harnessing machine learning for accurate treatment of overlapping opacity species in general circulation models

To understand high precision observations of exoplanets and brown dwarfs, we need detailed and complex general circulation models (GCMs) that incorporate hydrodynamics, chemistry, and radiation. For this study, we specifically examined the coupling between chemistry and radiation in GCMs and compared different methods for the mixing of opacities of different chemical species in the correlated-k assumption, when equilibrium chemistry cannot be assumed. We propose a fast machine learning method based on DeepSets (DS), which effectively combines individual correlated-k opacities (k-tables). We evaluated the DS method alongside other published methods such as adaptive equivalent extinction (AEE) and random overlap with rebinning and resorting (RORR). We integrated these mixing methods into our GCM (expeRT/MITgcm) and assessed their accuracy and performance for the example of the hot Jupiter HD~209458 b. Our findings indicate that the DS method is both accurate and efficient for GCM usage, whereas RORR is too slow. Additionally, we observed that the accuracy of AEE depends on its specific implementation and may introduce numerical issues in achieving radiative transfer solution convergence. We then applied the DS mixing method in a simplified chemical disequilibrium situation, where we modeled the rainout of TiO and VO, and confirmed that the rainout of TiO and VO would hinder the formation of a stratosphere. To further expedite the development of consistent disequilibrium chemistry calculations in GCMs, we provide documentation and code for coupling the DS mixing method with correlated-k radiative transfer solvers. The DS method has been extensively tested to be accurate enough for GCMs; however, other methods might be needed for accelerating atmospheric retrievals.

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arXiv2023

chemcomp: Modeling the chemical composition of planets formed in protoplanetary disks

Future observations of exoplanets will hopefully reveal detailed constraints on planetary compositions. Recently, we have developed and introduced chemcomp (Schneider & Bitsch 2021a), which simulates the formation of planets in viscously evolving protoplanetary disks by the accretion of pebbles and gas. The chemical composition of planetary building blocks (pebbles and gas) is traced by including a physical approach of the evaporation and condensation of volatiles at evaporation lines. We have now open-sourced the chemcomp code to enable comparisons between planet formation models and observational constraints by the community. The code can be found at https://github.com/AaronDavidSchneider/chemcomp, is easy to use (using configuration files) and comes with a detailed documentation and examples.

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arXiv2023

Constraining the formation history of the HAT-P-11 system by atmospheric abundances

The chemical fingerprint of a planet reveals information about its formation history regarding when and where it formed. The water content of a planet can help to constrain its formation pathway: If the planet formed in the outer regions of the disk and migrated inward, it will be water-rich due to the accretion of water-ice-rich solids. Conversely, formation in the inner disk, where water-ice is not available, will result in a smaller atmospheric water content due to the limited accretion of water vapor. This process complicates with the presence of gap-opening giant planets. A gas giant exerts a pressure bump exterior to its orbit, preventing the further influx of pebbles into the inner system, resulting in a water-poor environment. The different formation scenarios can help to constrain the formation of the HAT-P-11 system, which contains an inner sub-Neptune with a mass of 23.4 $\mathrm{M_{\oplus}}$ and substellar water abundances ($X_\mathrm{H_2O} \approx 0.11$), as well as an outer giant planet orbiting exterior to the water-ice line. Our planet formation model encompasses planetary growth through pebble and gas accretion, along with a pebble drift and evaporation module that enables us to track the chemical composition of the disk and the planets. We find that the presence of the gas giant is necessary to block water-ice-rich material, resulting in a substellar water content for the inner sub-Neptune. If the giant planet forms too early, not enough solid material can enter the inner disk regions, preventing the efficient growth of the inner planet. This highlights the importance of the timing of giant planet formation to explain the inner system structure. Our simulations predict a roughly stellar C/O ratio with superstellar C/H and O/H for HAT-P-11b, providing constraints for future observations that are essential for gaining a more detailed understanding of its formation.

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arXiv2023

Evidence of Radius Inflation in Radiative GCM Models of WASP-76b due to the Advection of Potential Temperature

Understanding the discrepancy between the radii of observed hot Jupiters and standard 'radiative-convective' models remains a hotly debated topic in the exoplanet community. One mechanism which has been proposed to bridge this gap, and which has recently come under scrutiny, is the vertical advection of potential temperature from the irradiated outer atmosphere deep into the interior, heating the deep, unirradiated, atmosphere, warming the internal adiabat, and resulting in radius inflation. Specifically, a recent study which explored the atmosphere of WASP-76b using a 3D, non-grey, GCM suggested that their models lacked radius inflation, and hence any vertical enthalpy advection. Here we perform additional analysis of these, and related, models, focusing on an explicit analysis of vertical enthalpy transport and the resulting heating of the deep atmosphere compared with 1D models. Our results indicate that, after any evolution linked with initialisation, all the WASP-76b models considered here exhibit significant vertical enthalpy transport, heating the deep atmosphere significantly when compared with standard 1D models. Furthermore, comparison of a long time-scale (and hence near steady-state) model with a Jupiter-like internal-structure model suggests not only strong radius-inflation, but also that the model radius, $1.98 \mathrm{R_{J}}$, may be comparable with observations ($1.83\pm0.06 \mathrm{R_{J}}$). We thus conclude that the vertical advection of potential temperature alone is enough to explain the radius inflation of WASP-76b, and potentially other irradiated gas giants, albeit with the proviso that the exact strength of the vertical advection remains sensitive to model parameters, such as the inclusion of deep atmospheric drag.

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arXiv2022

No evidence for radius inflation in hot Jupiters from vertical advection of heat

Understanding the radiative-dynamical coupling between upper photosphere and deeper atmosphere is a key in understanding the abnormal large radii of hot Jupiters. One needs very long integration times of 3D general circulation models (GCMs) with self consistent radiative transfer to achieve a better understanding of the feedback process between dynamics and radiation. We here present the longest 3D non-gray GCM study (86000 d) of an ultra hot Jupiter (WASP-76 b) published to this date that reached a final converged state. Furthermore, we present a method that can be used to accelerate the path towards temperature convergence in the deep atmospheric layers. We find that the final converged temperature profile is cold in the deep atmospheric layers, lacking any sign of vertical transport of potential temperature by large scale atmospheric motions. We thus conclude that the coupling between radiation and dynamics alone is not sufficient to explain the abnormal large radii of inflated hot gas giants.

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arXiv2022

Exploring the deep atmospheres of HD 209458b and WASP-43b using a non-gray general circulation model

Simulations with a 3D general circulation model (GCM) suggest that one potential driver behind the observed radius inflation in hot Jupiters may be the downward advection of energy from the highly irradiated photosphere into the deeper layers. Here, we compare dynamical heat transport within the non-inflated hot Jupiter WASP-43b and the canonical inflated hot Jupiter HD 209458b, with similar effective temperatures. We investigate to what extent the radiatively driven heating and cooling in the photosphere (at pressures smaller than 1 bar) influence the deeper temperature profile (at pressures between 1 to 700 bar). Our simulations with the new non-gray 3D radiation-hydrodynamical model expeRT/MITgcm show that the deep temperature profile of WASP-43b is associated with a relatively cold adiabat. The deep layers of HD 209458b, however, do not converge and remain nearly unchanged regardless of whether a cold or a hot initial state is used. Furthermore, we show that different flow structures in the deep atmospheric layers arise. There, we find that WASP-43b exhibits a deep equatorial jet, driven by the relatively fast tidally locked rotation of this planet (0.81 days), as compared to HD 209458b (3.47 days). However, by comparing simulations with different rotation periods, we find that the resulting flow structures only marginally influence the temperature evolution in the deep atmosphere, which is almost completely dominated by radiative heating and cooling. Furthermore, we find that the evolution of deeper layers can influence the 3D temperature structure in the photosphere of WASP-43b. Thus, dayside emission spectra of WASP-43b may shed more light onto the dynamical processes occurring at greater depths.

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arXiv2022

How drifting and evaporating pebbles shape giant planets III: The formation of WASP-77A b and $τ$ Boötis b

Atmospheric abundances are thought to constrain the planet formation pathway, because different species evaporate at different temperatures leaving distinct signatures in the accreted atmosphere. The planetary C/O ratio is thought to constrain the planet formation pathway, because of the condensation sequence of H$_2$O, CO$_2$, CH$_4$, and CO, resulting in an increase of the gas phase C/O ratio with increasing distance. Here we use a disc evolution model including pebble growth, drift and evaporation coupled with a planet formation model that includes pebble and gas accretion as well as planet migration to compute the atmospheric compositions of giant planets. We compare our results to the recent observations of the hot Jupiters WASP-77A b and $τ$ Boötis b, which feature sub-solar and super-solar C/H and O/H values, respectively. Our simulations reproduce these measurements and show that giants like WASP-77A b should start to form beyond the CO$_2$ evaporation front, while giants like $τ$ Boötis b should originate from beyond the H$_2$O line. Our model allows the formation of sub- and super-solar atmospheric compositions. However simulations without pebble evaporation can not reproduce the super-solar C/H and O/H ratios of $τ$ Boötis b's atmosphere without additional accretion of solids. We identify the $α$ viscosity parameter of the disc as a key ingredient, because the viscosity drives the inward motion of volatile enriched vapor, responsible for the accretion of gaseous C and O. Depending on the planet's migration history order-of-magnitude differences in atmospheric C/H and O/H are expected. Our simulations also predict super-solar N/H for $τ$ Boötis b and solar N/H for WASP-77A b. We conclude that pebble evaporation is a key ingredient to explain the variety of exoplanet atmospheres, because it can explain both, sub- and super-solar atmospheric abundances.

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arXiv2021

How drifting and evaporating pebbles shape giant planets II: Volatiles and refractories in atmospheres

Upcoming studies of extrasolar gas giants will give precise insights into the composition of planetary atmospheres with the ultimate goal to link it to the formation history of the planet. Here, we investigate how drifting and evaporating pebbles that enrich the gas phase of the disk influence the chemical composition of growing and migrating gas giants. To achieve this goal, we perform semi analytical 1D models of protoplanetary disks including viscous evolution, pebble drift and evaporation to simulate the growth of planets from planetary embryos to Jupiter mass objects by the accretion of pebbles and gas while they migrate through the disk. The gas phase of the protoplanetary disk is enriched due to the evaporation of inward drifting pebbles crossing evaporation lines, leading to the accretion of large amounts of volatiles into the planetary atmosphere. As a consequence, gas accreting planets are enriched in volatiles (C, O, N) compared to refractories (e.g., Mg, Si, Fe) by up to a factor of 100, depending on the chemical species, its exact abundance and volatility as well as the disk's viscosity. A simplified model for the formation of Jupiter reveals that its nitrogen content can be explained by inward diffusing nitrogen rich vapor, implying that Jupiter does not need to form close to the N2 evaporation front as indicated by previous simulations. However, our model predicts a too low oxygen abundance for Jupiter, implying either Jupiter's migration across the water ice line or an additional accretion of solids into the atmosphere, which can also increase Jupiter's carbon abundance. The accretion of solids will increase the refractory to volatile ratio in planetary atmospheres substantially. We thus conclude that the volatile to refractory ratio can place a strong constraint on planet formation theories making it an important target for future observations.

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arXiv2021

How drifting and evaporating pebbles shape giant planets I: Heavy element content and atmospheric C/O

Recent observations of extrasolar gas giants suggest super-stellar C/O ratios in planetary atmospheres, while interior models of observed extrasolar giant planets additionally suggest high heavy element contents. Furthermore, recent observations of protoplanetary disks revealed super-solar C/H ratios, which are explained by inward drifting and evaporating pebbles, enhancing the volatile content of the disk. We investigate how the inward drift and evaporation of volatile rich pebbles influences the atmospheric C/O ratio and heavy element content of giant planets growing by pebble and gas accretion. To achieve this goal, we perform semi analytical 1D models of protoplanetary disks including the treatment of viscous evolution and heating, pebble drift and simple chemistry to simulate the growth of planets from planetary embryos to Jupiter mass objects by accretion of pebbles and gas while they migrate through the disk. Our simulations show that the composition of the planetary gas atmosphere is dominated by the accretion of vapour, originating from inward drifting evaporating pebbles. This process allows the giant planets to harbour large heavy element contents. In addition, our model reveals that giant planets originating further away from the central star have a higher C/O ratio on average due to the evaporation of methane rich pebbles in the outer disk. These planets can then also harbour super-solar C/O ratios, in line with exoplanet observations. However, planets formed in the outer disk harbour a smaller heavy element content, due to a smaller vapour enrichment of the outer disk. Our model predicts that giant planets with low/large atmospheric C/O should harbour a large/low total heavy element content. We further conclude that the inclusion of pebble evaporation at evaporation lines is a key ingredient to determine the heavy element content and composition of giant planets.

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arXiv2021

Dry or water world? How the water contents of inner sub-Neptunes constrain giant planet formation and the location of the water ice line

In the pebble accretion scenario, the pebbles that form planets drift inward from the outer disk regions, carrying water ice with them. At the water ice line, the water ice on the inward drifting pebbles evaporates and is released into the gas phase, resulting in water-rich gas and dry pebbles that move into the inner disk regions. Large planetary cores can block the inward drifting pebbles by forming a pressure bump outside their orbit in the protoplanetary disk. Depending on the relative position of a growing planetary core relative to the water ice line, water-rich pebbles might be blocked outside or inside the water ice line. Pebbles blocked outside the water ice line do not evaporate and thus do not release their water vapor into the gas phase, resulting in a dry inner disk, while pebbles blocked inside the water ice line release their water vapor into the gas phase, resulting in water vapor diffusing into the inner disk. As a consequence, close-in sub-Neptunes that accrete some gas from the disk should be dry or wet, respectively, if outer gas giants are outside or inside the water ice line, assuming that giant planets form fast, as has been suggested for Jupiter in our Solar System. Alternatively, a sub-Neptune could form outside the water ice line, accreting a large amount of icy pebbles and then migrating inward as a very wet sub-Neptune. We suggest that the water content of inner sub-Neptunes in systems with giant planets that can efficiently block the inward drifting pebbles could constrain the formation conditions of these systems, thus making these sub-Neptunes exciting targets for detailed characterization (e.g., with JWST, ELT, or ARIEL). In addition, the search for giant planets in systems with already characterized sub-Neptunes can be used to constrain the formation conditions of giant planets as well.

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