High-temperature measurements of VUV-absorption cross sections of CO₂ and their application to exoplanets

¹ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
e-mail: venot@obs.u-bordeaux1.fr
² CNRS, LAB, UMR 5804, 33270 Floirac, France
³ Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583, Universités Paris Est Créteil (UPEC) et Paris Diderot (UPD), Créteil, France

Received: 18 December 2012
Accepted: 28 January 2013

Abstract

Context. Ultraviolet (UV) absorption cross sections are an essential ingredient of photochemical atmosphere models. Exoplanet searches have unveiled a large population of short-period objects with hot atmospheres, very different from what we find in our solar system. Transiting exoplanets whose atmospheres can now be studied by transit spectroscopy receive extremely strong UV fluxes and have typical temperatures ranging from 400 to 2500 K. At these temperatures, UV photolysis cross section data are severely lacking.

Aims. Our goal is to provide high-temperature absorption cross sections and their temperature dependency for important atmospheric compounds. This study is dedicated to CO₂, which is observed and photodissociated in exoplanet atmospheres. We also investigate the influence of these new data on the photochemistry of some exoplanets.

Methods. We performed these measurements with synchrotron radiation as a tunable VUV light source for the 115–200 nm range at 300, 410, 480, and 550 K. In the 195–230 nm range, we used a deuterium lamp and a 1.5 m Jobin-Yvon spectrometer and we worked at seven temperatures between 465 and 800 K. We implemented the measured cross section into a 1D photochemical model.

Results. For λ > 170 nm, the wavelength dependence of ln(σ_CO2(λ,T) × 1/(Q_v(T))) can be parametrized with a linear law. Thus, we can interpolate σ_CO2(λ,T) at any temperature between 300 and 800 K. Within the studied range of temperature, the CO₂ cross section can vary by more than two orders of magnitude. This, in particular, makes the absorption of CO₂ significant up to wavelengths as high as 230 nm, while it is negligible above 200 nm at 300 K.

Conclusions. The absorption cross section of CO₂ is very sensitive to temperature, especially above 160 nm. The model predicts that accounting for this temperature dependency of CO₂ cross section can affect the computed abundances of NH₃, CO₂, and CO by one order of magnitude in the atmospheres of hot Jupiter and hot Neptune. This effect will be more important in hot CO₂-dominated atmospheres.