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exo-PlanetarY high-Temperature Hydrocarbons by Emission and Absorption Spectroscopy – e-PYTHEAS

e-PYTHEAS

Exo-PlanetarY high-Temperature Hydrocarbons by Emission and Absorption Spectroscopy

Objectives and challenges

In order to analyze the large number of hot and complex spectra of brown dwarfs, exoplanets and AGBs available today and to infer<br />unique information on the chemistry and dynamics of their atmospheres, new spectroscopic parameters are needed. Water vapor<br />and methane are the main opacity sources detected in the infrared spectrum of these objects, with other hydrocarbons (acetylene,<br />ethene) and their isotopologues also contributing. Hot experimental hydrocarbon data are limited in frequency and temperature<br />coverage and accurate molecular models are lacking to generate complete high-temperature spectral linelists.

We are a consortium
of 5 French laboratories, internationally recognized for our expertise in theoretical, experimental or modeling aspects. We propose to
produce synthetic spectra validated by experiment in the 0.8-17µm region for 12CH4, 13CH4, CH3D, C2H2 and C2H4 up to 2500 K
and apply them to observations.

-Comparisons with cold, room and high-T experimental data show that first global theoretical ab initio based methane lists created for the range 0-13400 cm-1 for T up to 3000 K are suitable for high-resolution astrophysical applications. These data are freely distributed via TheoReTS information system (http://theorets.univ-reims.fr).
- Global effective analyses results for cold and hot bands in the 0–3300 cm–1 region are freely distributed through the MeCaSDa database:
vamdc.icb.cnrs.fr/PHP/methane.php).
- CRDS coupled to hypersonic flows is a promising novel technique for accessing the structure of very excited vibrational states by simplifying their rotational structure.
- Modelling and simulations have been performed to optimize the ground-based and space (in particular with ARIEL) future measurements by GSMA-Reims and LESIA

- Generate experimental and synthetic spectra in the 1-17 µm wavelength region for hydrocarbons and their isotopologues such as 12CH4, 13CH4, CH3D, C2H2, C2H4 and C2H6 up to 2500 K.
- Produce complete line lists without the current limitations in frequency and temperature ranges.
- Record new absorption and emission spectra at high temperature by unique techniques developed by IPR, LIPhy and GSMA.
- Produce global ab initio spectra predictions.
- Validate and empirically correct them using spectra measurements.
- Use the generated spectroscopic data to enhance the output of radiative transfer codes and improve our understanding of the processes involved in hot gaseous environments thanks to the analysis of a large amount of observations.
- Determine the role of the target species on the thermal structure of hot exoplanets and hence to a new understanding of the origin and evolution of exoplanetary atmospheres.

Several scientific publications and communications

The e-PYTHEAS project sits on the frontier between molecular physics, theoretical chemistry and astrophysics. It aims at enhancing our understanding of the radiative properties of hot gaseous media to allow for improved analysis and interpretation of the large amount of data available on the thousands of exoplanets and exoplanetary systems known to date. Our multidisciplinary approach – theoretical research validated by laboratory experiments and injected into models of the atmospheres of the giant gaseous planets – will help the scientific community to address essential questions on the formation and evolution of planetary systems.
Most of the exoplanets are hot and hence their spectra are very complex and contain numerous unassigned features. The large number of exoplanetary observations acquired via a combination of new, larger telescopes and space missions will therefore only be exploitable if and only if the adequate laboratory and theoretical data are made available for their analysis, without the current limitations.
Our consortium of five French laboratories and multiple associated partners proposes to improve the existing high-temperature spectroscopy data for several molecular species detected in exoplanets. Our strategy consists in producing experimental and theoretical data that are then applied to observations.
The provision of IR laboratory data of methane, acetylene ethylene and ethane, between 500 and 2500 K will help to refine thermal profiles and provide information on the gaseous composition, the hazes and their temporal variability. Currently available data are insufficient in many ways:
• All datasets are affected by large gaps below 1.65 µm, an important region in observations;
• High-temperature spectroscopic data cannot be extrapolated from low-T atmospheric databases such as HITRAN and GEISA. Such extrapolations fail to reproduce high J rovibrational transitions and hot band transitions involving highly excited vibrational levels and thus the question of the opacity calculations at high temperatures remains unanswered;
• Accurate molecular models are still missing to generate complete high-T line lists for hydrocarbon absorbers which dominate the spectrum of brown dwarfs, exoplanets and AGB stars and play a primary role in the physical chemistry of their outer atmospheres.
Based on state-of-the-art theoretical calculations and new models, extensive line lists (including positions, intensities, profiles, etc.) will be generated and validated by laboratory experiments: (i) emission spectroscopy at thermal equilibrium at T>500 K in the 1.4–17 µm region; (ii) Cavity Ring Down Spectroscopy (CRDS) in non-thermal equilibrium and hypersonic relaxation in the 1.5–1.7 µm region; (iii) direct absorption and CRDS at high sensitivity from 500 to 1000 K in the 1.26–1.71 µm region for weak line measurements; (iv) room temperature absorption down to 0.8 µm to reach highly excited vibrational states.
The feasibility of this challenging project is attested by our previous successful experience and established expertise in experiments, spectra analyses and theory: our low-T experimental data and ab initio predictions for methane, ethylene and their isotopologues are currently the most accurate available, while first CH4 line lists produced already at 2000 K above 2 µm show good agreement with observations.
At the end of our project, the scientific community will dispose of experimental and synthetic spectra in the 0.8–17 µm region for hydrocarbons and their isotopologues (12CH4, 13CH4, CH3D, C2H2, C2H4, C2H6) up to 2500 K. For each of these species, we will provide rovibrational assignments, H2-broadening coefficients and cross-sections, to be directly usable in radiative transfer codes.
Our consortium will establish the framework for a large international collaboration, which will benefit from a mutualisation of unique access to observations, special facilities, manpower, software and experimental means.

Project coordination

Athena Coustenis (Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LIPhy Laboratoire Interdisciplinaire de Physique
GSMA Groupe de Spectrometrie Moléculaire et Atmospherique
ULB Université Libre de Bruxelles - Service de Chimie Quantique et Photophysique
UCL University College London
ISMO Instittut des Sciences Moléculaires d'Orsay
UML University of Massachusetts Lowell
IPR Institut de Physique de Rennes
LESIA Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique
ICB CNRS / Laboratoire Interdisciplinaire Carnot de Bourgogne

Help of the ANR 655,050 euros
Beginning and duration of the scientific project: October 2016 - 48 Months

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