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Register a new userHigh-temperature measurements of VUV-absorption cross sections of CO2 and their application to exoplanets
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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. 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 implemented the measured cross section into a 1D photochemical model. For wavelengths > 170 nm, the wavelength dependence of ln(cross-section_CO2(wavelength, T)x1/Qv(T)) can be parametrized with a linear law. Thus, we can interpolate cross-section_CO2(wavelength, 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. The absorption cross section of CO2 is very sensitive to temperature. 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.
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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.
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.
A publicly available database of opacities for molecules of astrophysical interest, ExoMolOP, has been compiled for over 80 species, based on the latest line list data from the ExoMol, HITEMP and MoLLIST databases. These data are generally suitable for characterising high temperature exoplanet or cool stellar/substellar atmospheres, and have been computed at a variety of pressures and temperatures, with a few molecules included at room-temperature only from the HITRAN database. The data are formatted in different ways for four different exoplanet atmosphere retrieval codes; ARCiS, TauREx, NEMESIS and petitRADTRANS, and include both cross-sections (at R~=~$frac{lambda}{Delta lambda}$~=~15,000) and k-tables (at R~=~$frac{lambda}{Delta lambda}$~=~1000) for the 0.3~-~50$mu$m wavelength region. Opacity files can be downloaded and used directly for these codes. Atomic data for alkali metals Na and K are also included, using data from the NIST database and the latest line shapes for the resonance lines. Broadening parameters have been taken from the literature where available, or from those for a known molecule with similar molecular properties where no broadening data are available. The data are available from www.exomol.com.
The Milky Way Galaxy is literally teeming with exoplanets; thousands of planets have been discovered, with thousands more planet candidates identified. Terrestrial-like planets are quite common around other stars, and are expected to be detected in large numbers in the future. Such planets are the primary targets in the search for potentially habitable conditions outside the solar system. Determining the atmospheric composition of exoplanets is mandatory to understand their origin and evolution, as atmospheric processes play crucial roles in many aspects of planetary architecture. In this work we construct and exploit a 1D radiative transfer model based on the discrete-ordinates method in plane-parallel geometry. Radiative results are linked to a convective flux that redistributes energy at any altitude producing atmospheric profiles in radiative-convective equilibrium. The model has been applied to a large number (6250) of closely dry synthetic ce{CO2} atmospheres, and the resulting pressure and thermal profiles have been interpreted in terms of parameter variability. Although less accurate than 3D general circulation models, not properly accounting for e.g., clouds and atmospheric and ocean dynamics, 1D descriptions are computationally inexpensive and retain significant value by allowing multidimensional parameter sweeps with relative ease.
High resolution spectroscopy (HRS) has been used to detect a number of species in the atmospheres of hot Jupiters. Key to such detections is accurately and precisely modelled spectra for cross-correlation against the R$gtrsim$20,000 observations. There is a need for the latest generation of opacities which form the basis for high signal-to-noise detections using such spectra. In this study we present and make publicly available cross sections for six molecular species, H$_2$O, CO, HCN, CH$_4$, NH$_3$ and CO$_2$ using the latest line lists most suitable for low- and high-resolution spectroscopy. We focus on the infrared (0.95-5~$mu$m) and between 500-1500~K where these species have strong spectral signatures. We generate these cross sections on a grid of pressures and temperatures typical for the photospheres of super Earth, warm Neptunes and hot Jupiters using the latest H$_2$ and He pressure broadening. We highlight the most prominent infrared spectral features by modelling three representative exoplanets, GJ~1214~b, GJ~3470~b and HD~189733~b, which encompass a wide range in temperature, mass and radii. In addition, we verify the line lists for H$_2$O, CO and HCN with previous high resolution observations of hot Jupiters. However, we are unable to detect CH$_4$ with our new cross sections from HRS observations of HD~102195~b. These high accuracy opacities are critical for atmospheric detections with HRS and will be continually updated as new data becomes available.
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