ExoMol molecular line lists - XVII. The rotation-vibration spectrum of hot SO3

Sulphur trioxide (SO3) is a trace species in the atmospheres of the Earth and Venus, as well as being an industrial product and an environmental pollutant. A variational line list for 32S16O3, named UYT2, is presented containing 21 billion vibration–rotation transitions. UYT2 can be used to model infrared spectra of SO3 at wavelengths longwards of 2 μm (ν < 5000 cm−1) for temperatures up to 800 K. Infrared absorption cross-sections recorded at 300 and 500 C are used to validate the UYT2 line list. The intensities in UYT2 are scaled to match the measured cross-sections. The line list is made available in electronic form as supplementary data to this article and at www.exomol.com

Perspective: Accurate ro-vibrational calculations on small molecules

In what has been described as the fourth age of quantum chemistry, variational nuclear motion programs are now routinely being used to obtain the vibration-rotation levels and corresponding wavefunctions of small molecules to the sort of high accuracy demanded by comparison with spectroscopy. In this perspective, I will discuss the current state-of-the-art which, for example, shows that these calculations are increasingly competitive with measurements or, indeed, replacing them and thus becoming the primary source of data on key processes. To achieve this accuracy ab initio requires consideration of small effects, routinely ignored in standard calculations, such as those due to quantum electrodynamics. Variational calculations are being used to generate huge lists of transitions which provide the input for models of radiative transport through hot atmospheres and to fill in or even replace measured transition intensities. Future prospects such as the study of molecular states near dissociation, which can provide a link with low-energy chemical reactions, are discussed

TauREx3 PhaseCurve: A 1.5D Model for Phase-curve Description

In recent years, retrieval analysis of exoplanet atmospheres have been very successful, providing deep insights on the composition and the temperature structure of these worlds via transit and eclipse methods. Analysis of spectral phase-curve observations, which in theory provide even more information, are still limited to a few planets. In the next decade, new facilities such as NASA–James Webb Space Telescope and ESA-Ariel will revolutionize the field of exoplanet atmospheres and we expect that a significant time will be spent on spectral phase-curve observations. Most current models are still limited in their analysis of phase-curve data as they do not consider the planet atmosphere as a whole or they require large computational resources. In this paper we present a semi-analytical model that will allow computing exoplanet emission spectra at different phase angles. Our model provides a way to simulate a large number of observations while being only about four times slower than the traditional forward model for plane–parallel primary eclipse. This model, which is based on the newly developed TauREx 3 framework, will be further developed to allow for phase-curve atmospheric retrievals

Disentangling atmospheric compositions of K2-18 b with next generation facilities

Recent analysis of the planet K2-18 b has shown the presence of water vapour in its atmosphere. While the H_{2}O detection is significant, the Hubble Space Telescope (HST) WFC3 spectrum suggests three possible solutions of very different nature which can equally match the data. The three solutions are a primary cloudy atmosphere with traces of water vapour (cloudy sub-Neptune), a secondary atmosphere with a substantial amount (up to 50% Volume Mixing Ratio) of H_{2}O (icy/water world) and/or an undetectable gas such as N2 (super-Earth). Additionally, the atmospheric pressure and the possible presence of a liquid/solid surface cannot be investigated with currently available observations. In this paper we used the best fit parameters from Tsiaras et al. (Nat. Astron. 3, 1086, 2019) to build James Webb Space Telescope (JWST) and Ariel simulations of the three scenarios. We have investigated 18 retrieval cases, which encompass the three scenarios and different observational strategies with the two observatories. Retrieval results show that twenty combined transits should be enough for the Ariel mission to disentangle the three scenarios, while JWST would require only two transits if combining NIRISS and NIRSpec data. This makes K2-18 b an ideal target for atmospheric follow-ups by both facilities and highlights the capabilities of the next generation of space-based infrared observatories to provide a complete picture of low mass planets

A re-analysis of equilibrium chemistry in five hot Jupiters

Aims. Studies of chemistry and chemical composition are fundamental to exploring the formation histories of planets and planetary systems. We propose having another look at five targets to better determine their composition and the chemical mechanisms taking place in their atmospheres. We present a re-analysis of five hot Jupiters, combining multiple instruments and using Bayesian retrieval methods. We compare different combinations of molecules present in the simulated atmosphere and various chemistry types, as well as a range of cloud parametrizations. Following up on recent studies questioning the detection of Na and K in the atmosphere of HD 209458b as being potentially contaminated by stellar lines (when present), we study the impact on other retrieval parameters that may lead to misinterpretations of the presence of these alkali species. Methods. We used spatially scanned observations from the grisms G102 and G141 of the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope, with a wavelength coverage of ~0.8 to ~1.7 microns. We analyzed these data with the publicly available Iraclis pipeline. We added data from Space Telescope Imaging Spectrograph (STIS) observations to increase our wavelength coverage from ~0.4 to ~1.7 μm. We then performed a Bayesian retrieval analysis with the open-source TauREx using a nested sampling algorithm. We carried out the retrieval, taking into account molecular abundances that vary freely and then with equilibrium chemistry. We explored the influence of including Na and K on the retrieval of the molecules from the atmosphere. Results. Our data re-analysis and Bayesian retrieval are consistent with previous studies, but we do find small differences in the retrieved parameters. After all, Na and K have no significant impact on the properties of the planet atmospheres. Therefore, we present here our new best-fit models, taking into account molecular abundances that are allowed to vary freely as well as the equilibrium chemistry. This work is a preparation for a future addition of a more sophisticated representation of the chemistry involved, while taking into account disequilibrium effects such as vertical mixing and photochemistry

Detectability of Rocky-Vapour atmospheres on super-Earths with Ariel

Ariel will mark the dawn of a new era as the first large-scale survey characterising exoplanetary atmospheres with science objectives to address fundamental questions about planetary composition, evolution and formation. In this study, we explore the detectability of atmospheres vaporised from magma oceans on dry, rocky Super-Earths orbiting very close to their host stars. The detection of such atmospheres would provide a definitive piece of evidence for rocky planets but are challenging measurements with currently available instruments due to their small spectral signatures. However, some of the hottest planets are believed to have atmospheres composed of vaporised rock, such as Na and SiO, with spectral signatures bright enough to be detected through eclipse observations with planned space-based telescopes. In this study, we find that rocky super-Earths with a irradiation temperature of 3000 K and a distance from Earth of up to 20 pc, as well as planets hotter than 3500 K and closer than 50 pc, have SiO features which are potentially detectable in eclipse spectra observed with Ariel

An extensively validated C/H/O/N chemical network for hot exoplanet disequilibrium chemistry

Context: The reliability of one-dimensional disequilibrium chemistry models in hot exoplanet atmospheres depends on the chemical network used. To develop robust networks, we can rely on combustion studies that provide C/H/O/N chemical networks validated by vast amount of experimental data generated by the extensive research that has been done on hydrocarbon combustion and NOx formation in the last decades. // Aims: We aimed to build a new and updated C0–C2 chemical network to study the C/H/O/N disequilibrium chemistry of warm and hot exoplanet atmospheres that relies on extensively validated and recent state-of-the-art combustion networks. The reliability range of this network was aimed for conditions between 500–2500 K and 100–10−6 bar, with cautious extrapolation at lower temperature values. // Methods: We compared the predictions of seven networks over a large set of experiments, covering a wide range of conditions (pressures, temperatures, and initial compositions). To examine the consequences of this new chemical network on exoplanets atmospheric studies, we generated abundances profiles for GJ 436 b, GJ 1214 b, HD 189733 b, and HD 209458 b, using the 1D kinetic model FRECKLL and calculated the corresponding transmission spectra using TauREx 3.1. These spectra and abundance profiles have been compared with results obtained with our previous chemical network. // Results: Our new kinetic network is composed of 145 species and 1313 reactions mostly reversible. This network proves to be more accurate than our previous one for the tested experimental conditions. The nitrogen chemistry update is found to be very impactful on the abundance profiles, particularly for HCN, with differences up to four orders of magnitude. The CO2 profiles are also significantly affected, with important repercussions on the transmission spectrum of GJ 436 b. // Conclusions: These effects highlight the importance of using extensively validated chemical networks to gain confidence in our models predictions. As shown with CH2NH, the coupling between carbon and nitrogen chemistry combined with radicals produced by photolysis can have huge effects impacting the transmission spectra. This should be kept in mind when adding new elements like sulfur, as only adding a sub-mechanism neglects these coupling effects

Alfnoor: A Retrieval Simulation of the Ariel Target List

n this work, we present Alfnoor, a dedicated tool optimized for population studies of exoplanet atmospheres. Alfnoor combines the latest version of the retrieval algorithm, TauREx 3, with the instrument noise simulator ArielRad and enables the simultaneous retrieval analysis of a large sample of exo-atmospheres. We applied this tool to the Ariel list of planetary candidates and focus on hydrogen dominated, cloudy atmospheres observed in transit with the Tier-2 mode (medium Ariel resolution). As a first experiment, we randomized the abundances—ranging from 10^{−7} to 10^{−2}—of the trace gases, which include H_{2}O, CH_{4}, CO, CO_{2}, and NH_{3}. This exercise allowed us to estimate the detection limits for Ariel Tier-2 and Tier-3 modes when clouds are present. In a second experiment, we imposed an arbitrary trend between a chemical species and the effective temperature of the planet. A last experiment was run requiring molecular abundances being dictated by equilibrium chemistry at a certain temperature. Our results demonstrate the ability of Ariel Tier-2 and Tier-3 surveys to reveal trends between the chemistry and associated planetary parameters. Future work will focus on eclipse data, on atmospheres heavier than hydrogen, and will be applied also to other observatories

Cross-sections for heavy atmospheres: H₂O continuum

Most of the exoplanets detected up to now transit in front of their host stars, allowing for the generation of transmission spectra; the study of exoplanet atmospheres relies heavily upon accurate analysis of these spectra. Recent discoveries mean that the study of atmospheric signals from low-mass, temperate worlds are becoming increasingly common. The observed transit depth in these planets is small and more difficult to analyze. Analysis of simulated transmission spectra for two small, temperate planets (GJ 1214 b and K2-18 b) is presented, giving evidence for significant differences in simulated transit depth when the water vapor continuum is accounted for when compared to models omitting it. These models use cross-sections from the CAVIAR lab experiment for the water self-continuum up to 10,000 cm−1; these cross-sections exhibit an inverse relationship with temperature, hence lower-temperature atmospheres are the most significantly impacted. Including the water continuum strongly affects transit depths, increasing values by up to 60 ppm, with the differences for both planets being detectable with the future space missions Ariel and JWST. It is imperative that models of exoplanet spectra move toward adaptive cross-sections, increasingly optimized for H2O-rich atmospheres. This necessitates including absorption contribution from the water vapor continuum into atmospheric simulations