High-temperature measurements of VUV absorption cross sections with application to warm exoplanet atmospheres

1) Introduction

With current observational constraints, a large fraction of observed exoplanets whose atmospheres can be studied with spectroscopy orbits closely from their host stars, receiving a high stellar flux and having elevated equilibrium temperature (T_eq > 500 K). In particular, the high flux received in the ultraviolet (UV) can have important implications on the atmospheric composition of these planets. Indeed, UV photons can photodissociate atmospheric constituents and thus initiate photochemical reactions [1]. This impact of photochemistry on the composition of exoplanet atmospheres have been recently confirmed by observations of the warm gas giants WASP-39b and WASP-107b by JWST [2, 3]. Thus, the understanding of exoplanet atmospheres and the interpretation of their observations require the use of kinetic models that include photochemistry.

To implement photochemistry, 1D thermo-photochemical models use the UV absorption cross sections of molecules present in the atmospheres to calculate their photodissociation rates as a function of the altitude as well as the penetration depth of the UV photons through the atmosphere. To obtain accurate results, these models require data suitable for the conditions encounters in these environments often without equivalent in the solar system. However, the thermal dependency of the UV absorption cross sections of molecules is poorly known in the range of temperature observed in exoplanet atmospheres, which leads to uncertainties in the molecular abundances predicted by atmospheric models [4-6]. Here we present an experimental study of the thermal dependency of the UV absorption cross section of acetylene (C₂H₂) and ammonia (NH₃) from 296 to 793 K and for a large spectral domain ranging from 115 to 230 nm.

2) Material and Methods

The absorption spectra of gaseous acetylene were measured at high temperature using a new custom-made UV spectroscopy platform developed at the Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA, France). Briefly, the setup consists in a high-temperature absorption cell composed of a quartz tube closed at each extremity by MgF₂ windows mounted on stainless-steel flanges. The optical pathlength of the cell was 165 cm. The cell was installed in a furnace that can warm the cell up to 1373 K. For this study, spectra were measured at ambient temperature (296), 373, 473, 573, 673, and 773 K. We limited our measurements to a maximum temperature of 773 K because, at higher temperatures, we observed a fast thermal decomposition of C₂H₂ and NH₃.

UV spectra of C₂H₂ and NH₃ were measured from 115 to 230 nm using a McPherson 225 UHV monochromator. The absolute absorption cross sections of the molecules were calculated from the measured spectra using the Beer-Lambert law:

σ(λ, T) = (1/nL) × ln (I₀/I)

where σ(λ, T) is the absorption cross section (cm²) at a given wavelength λ and temperature T, L is the optical pathlength (cm), n the volume density of the gas in the cell (cm^-3), I₀ the intensity of the light transmitted through an empty cell and I the intensity of the light transmitted through the cell containing a density n of gas. Considering the gas as a perfect gas, the density of the gas n in the cell was calculated using the equation n = P / kb T where P is the pressure of the gas inside the cell (Pa), T the temperature (K), and kb the Boltzmann constant (J K^-1).

3) Results and Discussions

We found that the absolute absorption cross sections of C₂H₂ and NH₃ increase with the temperature. We observed an increase in the intensity of the continuum and a decrease in the intensity of the absorption bands as the gas temperature increased, which result in an overall increase of the absolute absorption cross section of these molecules with temperature.

To go further, we quantified the impact of these new data on the prediction of exoplanet atmospheres composition using the 1D thermo-photochemical model FRECKLL [7, 8]. After modelling a hypothetical exoplanet atmosphere, we found that the abundance profile of C₂H₂ is slightly modified when the absorption cross section of C₂H₂ measured at 773 K is used instead of the one at 296 K. This small variation agrees with the fact that the absorption cross section of C₂H₂ increases at the maximum by a factor 20 at 773 K compared to 296 K. In addition, these changes in the absorption cross sections affect the penetration of the actinic flux through the atmosphere from 150 to 230 nm, resulting in an attenuation of the flux at higher altitudes when using the cross section measured at 773 K instead of that measured at 296 K. Such changes in the penetration of the actinic flux could have consequences on the abundance of other absorbing species.

Acknowledgements

This work is supported by the ANR project ‘EXACT’ (ANR-21-CE49-0008-01), the Centre National d'Etudes Spatiales (CNES), and the CNRS/INSU Programme National de Planétologie (PNP).

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