Abstract
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. 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 performed these measurements for the 115 - 200 nm range at 300, 410, 480, and 550 K. In the 195 - 230 nm range, we worked at seven temperatures between 465 and 800 K. We found that the absorption cross section of CO2 is very sensitive to temperature, especially above 160 nm. 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. To investigate the influence of these new data on the photochemistry of exoplanets, we implemented the measured cross section into a 1D photochemical model. 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.
Highlights
IntroductionBIO Web of Conferences of these exoplanets, from the top of the atmosphere down to 100 mbar [11,12,13]
The first experiments dedicated to the determination of absorption cross sections of CO2 at temperatures different from 298 K were motivated by solar system planetary studies (Mars, Titan, Venus, primitive Earth) so were performed at lower temperatures [14,15,16,17]
Tunable VUV light between 115 and 200 nm was obtained from the synchrotron radiation facility BESSY in Berlin and measurements in the 195 - 230 nm range were performed at the Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA) in Créteil, France
Summary
BIO Web of Conferences of these exoplanets, from the top of the atmosphere down to 100 mbar [11,12,13] For these exoplanets and within this large pressure range, the temperature can vary roughly from 400 to 2500 K. Carbon dioxide (CO2) is one of these species It has been observed in extrasolar giant planet atmospheres [7, 8], but cross section measurements (σCO2 (λ, T )) are extremely sparse above room temperature. Some high-temperature measurements have been performed in the past but only at a few wavelengths [18, 19] These measurements were limited to a narrow range of wavelengths and do not provide complete spectra. They showed that the absorption of CO2 increases at high temperature. Spectra between 190 and 355 nm were obtained at very high temperatures (900-4500 K) by [20,21,22] and some of them fitted the strong temperature dependence of σCO2 (λ, T ) with an empirical function
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