VUV-absorption cross section of carbon dioxide from 150 to 800 K and applications to warm exoplanetary atmospheres
stronomy and Astrophysics
Most exoplanets detected so far have atmospheric T significantly higher than 300K. Often close to their star, they receive an intense UV photons flux that triggers important photodissociation processes. The T dependency of VUV absorption cross sections are poorly known, leading to an undefined uncertainty in atmospheric models. Similarly, data measured at low T similar to that of the high atmosphere of Mars, Venus, and Titan are often lacking. Our aim is to quantify the T dependency of the abs. cross section of important molecules in planetary atmospheres. We want to provide both high-resolution data at T prevailing in these media and a simple parameterization of the absorption in order to simplify its use in photochemical models. This study focuses on carbon dioxide. We performed experimental measurements of CO$_2$ absorption cross section with synchrotron radiation for the wavelength range (115--200nm). For longer wavelengths (195--230nm), we used a deuterium lamp and a 1.5m Jobin-Yvon spectrometer. We used these data in our 1D thermo-photochemical model in order to study their impact on the predicted atmospheric compositions. The cross section of CO$_2$ increases with T. It can be separated in two parts: a continuum and a fine structure superimposed on the continuum. The variation of the continuum of absorption can be represented by the sum of three gaussian functions. Using data at high T in thermo-photochemical models modifies significantly the abundance and the photodissociation rates of many species, in addition to CO$_2$, such as methane and ammonia. These deviations have an impact on synthetic transmission spectra, leading to variations of up to 5 ppm. We present a full set of HR ($\Delta \lambda$=0.03nm) absorption cross sections of CO$_2$ from 115 to 230nm for T ranging from 150 to 800K.
The authors wish to thank the anonymous referee for his very interesting comments. They also thank Gerd Reichard and Peter Baumgärtel for their excellent assistance during the synchrotron radiation beam time periods. The authors also acknowledge the financial supports of the European Commission Programme "Access to Research Infrastructures" for providing access to the synchrotron facility BESSY in Berlin, of the programme PIR EPOV, and that of the CNRS/INSU Programme National de Planétologie (PNP). O.V. acknowledges support from the KU Leuven IDO project IDO/10/2013, from the FWO Postdoctoral Fellowship programme, and from the Postdoctoral Fellowship programme of the Centre National d’Etudes Spatial (CNES). I.P.W. acknowledges support by the ERC project ExoLights (617119).
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.