High temperature VUV cross-section measurements for the study of hot exoplanets’ atmospheres: new line list and temperature dependance of A1Π-X1Σ+ CO transition, 3 ≤v’ ≤ 10 and v” = 0, 1

Introduction

The large diversity of planetary worlds has been firmly established, since the discovery of the first exoplanet in 1995. Some planets have the size of Jupiter and orbit at very close distances from their host star, making them “Hot Jupiter” with atmospheric temperatures as high as 2000 K. To understand how such diversity can be, we need more information about the physical and chemical characteristics of those planets. This is one of the main goals of the JWST mission and will be the main objective of the Ariel mission. The observations of these telescopes will be interpreted thanks to photo/thermochemical kinetics models, that calculate the atmospheric abundance profiles. Such models need input data like UV absorption cross sections of gases and, unfortunately most of them are badly constrained at high temperatures.

 

Experimental setup

To overcome this lack of knowledge in the temperature dependency of VUV absorption cross-sections, we have developed a new experiment at LISA to measure VUV absorption cross-sections of important molecules for atmospheric chemistry of warm exoplanets, such as CO, C₂H₂, NH₃ or HCN. The experimental setup is composed of a quartz cell inside an oven that can reach temperatures up to 1173K, a VUV monochromator with a deuterium lamp to produce the flux and a photomultiplier. With this experiment, we will be able to systematically determine the temperature variations of gas absorption cross-sections in the 110-300 nm wavelength domain.

 

Spectroscopic study

During the test of the spectroscopic platform, we have studied the absorption cross-section of CO in the A¹Π-X¹Σ⁺ transition between 68 000 and 78 000 cm^-1 (128 to 147 nm), at different temperatures between 300 and 800 K, thanks to experimental data acquired on the synchrotron facility SOLEIL. This work led us derive a new line list of the rovibrational transitions with 3≤v’≤10 et v”=0, 1. In addition, this line list was then used to determine the temperature of gas inside the oven. Indeed, one of the uncertainties occurring in this kind of laboratory measurements remains the determination of the temperature of gas inside the oven. It was the case when our group has determined some years ago the absorption cross-sections of CO₂ at high temperatures [1][2]. Here, our study shows that CO can be used as a direct temperature probe of the gas inside our oven. Finally, the obtained line list will be made available to the community for astrophysical studies (Poveda et al. in prep.).

 

Atmospheric model

These new data will be implemented in 1D kinetic models to simulate the composition of warm and hot exoplanets atmospheres. We use the ATMO code [3] coupled to a chemical scheme to describe the reactions occurring inside the atmosphere [4]. First, the code calculates the chemical composition at thermodynamic equilibrium. Then, stellar irradiation is introduced and the chemical composition is computed as a function of time, according to the continuity equation (1), to obtain the chemical composition of the atmosphere at steady state, with photochemical reactions and vertical mixing.

            (1)

 

Summary and Conclusions

In order to improve our knowledge of hot exoplanets’ atmospheres, we have developed a new VUV spectroscopic platform at LISA. We have started to perform measurements on absorption cross-sections of CO and C₂H₂ at different temperatures, and thanks to spectroscopic studies we are able to analyse these data. The first study was made on CO and this led us to draw up a new line list of the rovibrational transitions of CO in the VUV field. At the end, this line list will be made available to the community for astrophysical study.

 

References

[1] High-temperature measurements of VUV-absorption cross sections of CO2 and their application to exoplanets, O.Venot et al., Astronomy & Astrophysics, 2013.

[2] VVV-absorption cross section of carbon dioxide from 150 to 800 K and applications to warm exoplanetary atmospheres, O.Venot et al., Astronomy & Astrophysics, 2018.

[3] Fingering convection and cloudless models for cool brown dwarf atmospheres, P.Tremblin et al., The Astrophysical Journal Letters, 804:L17 (6pp), 2015 May.

[4] New chemical scheme for giant planet thermochemistry, O.Venot et al., Astronomy & Astrophysics, 2020.