Toward a multidimensional analysis of transmission spectroscopy. Part III: Modelling 2D effects in retrievals with TauREx

New-generation spectrographs dedicated to the study of exoplanetary atmospheres require a high accuracy in the atmospheric models to better interpret the input spectra. Thanks to space missions, the observed spectra will cover a large wavelength range from visible to mid-infrared with an higher precision compared to the old-generation instrumentation, revealing complex features coming from different regions of the atmosphere. For hot and ultra hot Jupiters (HJs and UHJs), the main source of complexity in the spectra comes from thermal and chemical differences between the day and the night sides. In this context, one-dimensional plane parallel retrieval models of atmospheres may not be suitable to extract the complexity of such spectra. In addition, Bayesian frameworks are computationally intensive and prevent us from using complete three-dimensional self-consistent models to retrieve exoplanetary atmospheres. We propose the TauREx 2D retrieval code, which uses two-dimensional atmospheric models as a good compromise between computational cost and model accuracy to better infer exoplanetary atmospheric characteristics for the hottest planets. TauREx 2D uses a 2D parametrization across the limb which computes the transmission spectrum from an exoplanetary atmosphere assuming azimuthal symmetry. It also includes a thermal dissociation model of various species. We demonstrate that, given an input observation, TauREx 2D mitigates the biases between the retrieved atmospheric parameters and the real atmospheric parameters. We also show that having a prior knowledge on the link between local temperature and composition is instrumental in inferring the temperature structure of the atmosphere. Finally, we apply such a model on a synthetic spectrum computed from a GCM simulation of WASP-121b and show how parameter biases can be removed when using two-dimensional forward models across the limb.Comment: 16 pages, 16 figures. Accepted for publication in Astronomy & Astrophysic

The effect of a small amount of hydrogen in the atmosphere of ultrahot magma-ocean planets: atmospheric composition and escape

Here we investigate how small amounts of hydrogen (much smaller than the mass of the exoplanet) above a magma ocean on a rocky exoplanet may modify the atmospheric chemistry and atmospheric escape.We use a chemical model of a magma ocean coupled to a gas equilibrium code. An energy-limited model is used to compute atmospheric escape. The composition of the vapor above a magma ocean is drastically modified by hydrogen, even for very modest amounts of H ($\ll 10^{-6}$ planetary mass). Hydrogen consumes much of the O$_2$(g), which, in turn, promotes the evaporation of metals and metal oxides (SiO, Mg, Na, K, Fe) from the magma ocean. Vast amounts of H$_2$O are produced by the same process. At high hydrogen pressures, new hydrogenated species such as SiH$_4$ form in the atmosphere. In all cases, H, H$_2$, and H$_2$O are the dominant nonmetal-bearing volatile species. Sodium is the dominant atmospheric metal-bearing species at T$<$ 2000K and low H content, whereas Fe is dominant at high H content and low temperature, while SiO predominates at T>3000 K. We find that the atmospheric Mg/Fe, Mg/Si, and Na/Si ratios deviate from those in the underlying planet and from the stellar composition. As such, their determination may constrain the planet's mantle composition and H content. As the presence of hydrogen promotes the evaporation of silicate mantles, it is conceivable that some high-density, irradiated exoplanets may have started life as hydrogen-bearing planets and that part of their silicate mantle evaporated (up to a few $10 \%$ of Si, O, and Fe) and was subsequently lost owing to the reducing role of H. Even very small amounts of H can alter the atmospheric composition and promote the evaporation to space of heavy species derived from the molten silicate mantle of rocky planets.Comment: Accepted for publication in A&

Factorisation symbolique hiérarchique pour des matrices creuses

International audienc

Evaporation of a magma ocean in presence of hydrogen & observational implications

https://baas.aas.org/pub/2023n8i410p04International audienc

Evaporation of a magma ocean in presence of hydrogen & observational implications

https://baas.aas.org/pub/2023n8i410p04International audienc

Vers une factorisation symbolique hiérarchique de rang faible pour des matrices creuses

National audienceLes algorithmes hiérarchiques basés sur des techniques de compression de rang faible ont ré-volutionné les méthodes de résolution de systèmes linéaires denses à l'aube du XXIème siècle en réduisant considérablement les coûts de calcul. Toutefois, leur application au traitement de systèmes linéaires creux (comportant de nombreux éléments nuls), qui sont le type de pro-blèmes apparaissant le plus souvent au coeur des simulations numériques, reste aujourd'hui un challenge majeur auquel s'attellent à la fois la communauté des matrices hiérarchiques et celle des matrices creuses. À cet effet, une première classe d'approche a été développée par la communauté des matrices hiérarchiques pour exploiter la structure creuse des matrices. Si le point fort de ces méthodes est que l'algorithme résultant reste hiérarchique, celles-ci ne par-viennent pas à exploiter certains zéros comme le font naturellement les méthodes classiques de factorisation de systèmes linéaires creux. À l'opposé, du fait qu'une factorisation creuse peut être vue comme une séquence de plus petites opérations denses, la communauté des matrices creuses a exploré cette propriété pour introduire des techniques hiérarchiques au sein de ces opérations élémentaires. Cependant, l'algorithme résultant perd la propriété fondamentale des algorithmes hiérarchiques, dans la mesure où la hiérarchie de compression est seulement locale. Dans cet article, nous introduisons un nouvel algorithme, effectuant une factorisation symbolique creuse hiérarchique qui permet d'exploiter efficacement l'ensemble des zéros de la matrice creuse et de ses facteurs tout en préservant une structure hiérarchique globale. Nous montrons expérimentalement que cette nouvelle approche permet d'obtenir à la fois un nombre réduit d'opérations (du fait de son caractère hiérarchique) et un nombre d'éléments non nuls aussi réduit qu'une méthode creuse (grâce au recours à une factorisation symbolique)

Toward a multidimensional analysis of transmission spectroscopy: I. Computation of transmission spectra using a 1D, 2D, or 3D atmosphere structure

International audienceConsidering the relatively high precision that will be reached by future observatories, it has recently become clear that one dimensional (1D) atmospheric models, in which the atmospheric temperature and composition of a planet are considered to vary only in the vertical, will be unable to represent exoplanetary transmission spectra with a sufficient accuracy. This is particularly true for warm to (ultra-) hot exoplanets because the atmosphere is unable to redistribute all the energy deposited on the dayside, creating a strong thermal and often compositional dichotomy on the planet. This situation is exacerbated by transmission spectroscopy, which probes the terminator region. This is the most heterogeneous region of the atmosphere. However, if being able to compute transmission spectra from 3D atmospheric structures (from a global climate model, e.g.) is necessary to predict realistic observables, it is too computationally expensive to be used in a data inversion framework. For this reason, there is a need for a medium-complexity 2D approach that captures the most salient features of the 3D model in a sufficiently fast implementation. With this in mind, we present a new open-source documented version of Pytmosph3R that handles the computation of transmission spectra for atmospheres with up to three spatial dimensions and can account for time variability. Taking the example of an ultrahot Jupiter, we illustrate how the changing orientation of the planet during the transit can allow us to probe the horizontal variations in the atmosphere. We further implement our algorithm in TauREx to allow the community to perform 2D retrievals. We describe our extensive cross-validation benchmarks and discuss the accuracy and numerical performance of each model

The effect of a small amount of hydrogen in the atmosphere of ultrahot magma-ocean planets: Atmospheric composition and escape

International audienceContext. Ultrahot (>1500 K) rocky exoplanets may be covered by a magma ocean, resulting in the formation of a vapor rich in rocky components (e.g., Mg, Si, Fe) with a low total pressure and high molecular mass. However, exoplanets may have also captured a significant amount of hydrogen from the nebular gas during their formation. Ultrahot rocky exoplanets around the Fulton gap (~1.8 R⊕) are sufficiently large to have retained some fraction of their primordial hydrogen atmosphere. Aims: Here, we investigate how small amounts of hydrogen (much smaller than the mass of the planet) above a magma ocean may modify the atmospheric chemistry and its tendency to thermally escape. Methods: We use a chemical model of a magma ocean coupled to a gas equilibrium code (that includes hydrogen) to compute the atmospheric composition at thermodynamical equilibrium for various H contents and temperatures. An energy-limited model is used to compute atmospheric escape and is scaled to consider H-rich and H-poor atmospheres. Results: The composition of the vapor above a magma ocean is drastically modified by hydrogen, even for very modest amounts of H (≪10−6 planetary mass). Hydrogen consumes much of the O2(g), which, in turn, promotes the evaporation of metals and metal oxides (SiO, Mg, Na, K, Fe) from the magma ocean. Vast amounts of H2O are produced by the same process. At high hydrogen pressures, new hydrogenated species such as SiH4 form in the atmosphere. In all cases, H, H2, and H2O are the dominant nonmetal-bearing volatile species. Sodium is the dominant atmospheric metal-bearing species at T 3000 K. We find that the atmospheric Mg/Fe, Mg/Si, and Na/Si ratios deviate from those in the underlying planet and from the stellar composition. As such, their determination may constrain the planet's mantle composition and H content. As the presence of hydrogen promotes the evaporation of silicate mantles, it is conceivable that some high-density, irradiated exoplanets may have started life as hydrogen-bearing planets and that part of their silicate mantle evaporated (up to a few 10% of Si, O, and Fe) and was subsequently lost owing to the reducing role of H. Conclusions: Even very small amounts of H can alter the atmospheric composition and promote the evaporation to space of heavy species derived from the molten silicate mantle of rocky planets. Through transit spectroscopy, the measurement of certain elemental ratios, along with the detection of atmospheric water or hydrogen, may help to determine the nature of a surface magma ocean

Toward a multidimensional analysis of transmission spectroscopy: II. Day-night-induced biases in retrievals from hot to ultrahot Jupiters

International audienceHot Jupiters (HJs) are very good targets for transmission spectroscopy analysis. Their atmospheres have a large scale height, implying a high signal-to-noise ratio. As these planets orbit close to their stars, they often present strong thermal and chemical heterogeneities between the day- and nightside of their atmosphere. For the hottest of these planets, the thermal dissociation of several species occurs in their atmospheres, which leads to a stronger chemical dichotomy between the two hemispheres. It has already been shown that the current retrieval algorithms, which are using 1D forward models, find biased molecular abundances in ultrahot Jupiters. Here, we quantify the effective temperature domain over which these biases are present. We used a set of 12 simulations of typical HJs from Teq = 1000 K to Teq = 2100 K performed with the substellar and planetary atmospheric radiation and circulation global climate model and generate transmission spectra that fully account for the 3D structure of the atmosphere with Pytmosph3R. These spectra were then analyzed using the 1D TauREx retrieval code. We find that for James Webb Space Telescope like data, accounting for nonisothermal vertical temperature profiles is required over the whole temperature range. We further find that 1D retrieval codes start to estimate incorrect parameter values for planets with equilibrium temperatures greater than 1400 K if there are absorbers in the visible (such as TiO and VO, e.g.) that are able to create a hot stratosphere. The high temperatures at low pressures indeed entail a thermal dissociation of species that creates a strong chemical day-night dichotomy. As a byproduct, we demonstrate that when synthetic observations are used to assess the detectability of a given feature or process using a Bayesian framework (e.g., by comparing the Bayesian evidence of retrievals with different model assumptions), it is valid to use nonrandomized input data as long as the anticipated observational uncertainties are correctly taken into account