Exomol: Computational spectroscopy of Exoplanets

Exomol: Computational spectroscopy of Exoplanets

Molecular spectra play a critical role for the retrieval of atmospheres of exoplanets and cool starts. However, the lack of laboratory data on many of molecular species severely limits models for objects as diverse as Jupiter, exoplanets and brown dwarfs. The UCL ExoMol team is world leader in providing molecular line lists for exoplanet and other hot atmospheres. The ExoMol procedure uses a mixture of ab initio calculations and available laboratory data. These line lists form the input for opacity models for cool stars and brown dwarfs as well as for radiative transport models involving exoplanets. So far ExoMol has collected molecular opacities for more than 50 molecules (130 isotopologues): 30 line lists have been generated using the ExoMol methodology and other line lists were taken from external sources or from our work predating the ExoMol project.

Ethylene and Acetylene are important absorber in hot Jupiter exoplanets, brown dwarfs and cool carbon stars. As part of our Hydrocarbons DiRAC project, we have new line lists for these molecules, each of which contain over 30 billion transitions. In order to accomplish this data intensive task, some of the UK’s most advanced supercomputers have been used, provided by the DiRAC project. Even larger line lists have been completed. For example, the methyl chloride’s line list contains over 300 billion lines.

Many of those exoplanets discovered thus far are categorized as rocky objects with an atmosphere. Most of these objects are however hot due to their short orbital period. Models suggest that water is the dominant species in their atmospheres. The hot temperatures are expected to turn these atmospheres into a (high pressure) steam bath containing remains of melted rock. The spectroscopy of these hot rocky objects will be very different from that of cooler objects or hot gas giants. Molecules suggested to be important for the spectroscopy of these objects are reviewed together with the current status of the corresponding spectroscopic data. We have started building a comprehensive database of linelist/cross sections applicable for atmospheric models of rocky super-Earths as part of the ExoMol project.

An efficient usage of line lists comprising billions of lines is extremely challenging. To address this challenge of Big Data, we have developed a new Fortran 2003 program ExoCross. ExoCross takes line lists as input and returns pressure- and temperature-dependent cross sections as well a variety of other derived molecular properties which depend on the underlying spectroscopic data. These include state-dependent lifetimes, temperature-dependent cooling functions, and thermodynamic properties such as partition functions and specific heats and is designed to work with the recently proposed method of super-lines. It is capable of simulating non-LTE spectra using a simple two-temperature approach.

Supported by the DiRAC high performance facilities, we have organised and run a workshop “Digital Exoplanets” in Prague, January 2019 (www.digitalexoplanets.org). The workshop gathered over 50 computational exoplanetary scientists (with more than half career researchers) and focused on the data for atmospheric models of exoplanets and cool stars. The main theme of the workshop was the open access to the data and computer codes. This was the first workshops of this kind, focusing not only on observations and theory, but mostly on ‘how to actually run these codes’. The participants had the opportunity to learn how to use (install, run, debug etc) different main stream atmospheric and spectroscopic codes during the hands-on sessions, which were the central part of the workshop. These practical sessions were run on the resources provided by DiRAC team, with the help of the DiRAC team, which is greatly appreciated.