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ExoMol: molecular line lists for exoplanet atmospheres

Final Report Summary - EXOMOL (ExoMol: molecular line lists for exoplanet atmospheres)

The ExoMol project aimed provide complete laboratory data for analysing the how molecules in the atmospheres of hot astronomical, objects particularly exoplanets, cools stars and brown dwarfs absorb and emit light. This information is vital for interpreting the spectroscopic signals from these bodies that reach earth. While the primary focus of the ExoMol project was astrophysics, it is recognised that the data provided has a significant number of applications to technological problems such as combustion. The aim of the project was to make the data as widely available as possible. For this reason web domain was created and used.

The methodology used by ExoMol is primarily theoretical. For each molecule studied, the underlying quantum mechanical equations of the problem were constructed and solved. This was done in two steps: first by solving for the motion of the electrons and then the nuclei. The lists of molecular transitions produced (line lists) comprise transition frequencies and transition probabilities (in the form of Einstein A coefficients). To get accurate frequencies the problem is tuned using available experimental data; however the computed probabilities are usually as accurate as the measured ones which in any case are often not available. Besides improvements to existing computer programs, significant new methods were developed and programmed. In particular for example a completely general method was developed to calculate the fully-coupled vibronic spectra of diatomic molecules and a hybrid variational-perturbation theory approach to computing the spectra of large molecules was formulated, tested and implemented. Novel use was also made of graphical processing units which greatly improved the efficiency with which we computed huge line lists: the polyatomic line lists each contain 10 or more billion line.

The resulting line lists are huge with those for 5 of the systems studied as part of the ExoMol exceeding 10 billion transitions. The resulting line lists have been published as series of papers in Monthly Notices of the Royal astronomical Society, with a significant number of ancillary papers describing underlying physics and our other scientific outcomes published elsewhere. So far new line lists have been published for 23 molecules including key species such as methane and silicon monoxide. These line lists have been incorporated in a number of codes designed to model exoplanets such as hot Jupiters, brown dwarfs and cools stars. They are also being used to simulate the spectra that would observable by various planned exoplanet characterisation space missions.

Models of spectra of brown dwarf stars performed using our new 10 billion line methane line list showed that quantitative agreement in contrast to previous models using the best available data which failed to reproduce significant features in the observed spectrum. The ExoMol database provides a repository where these linelists plus related data such as cross sections, partition functions, pressure broadening parameters and lifetimes can be found. These data are being widely used by astronomers and by other scientists for a whole variety different problems, not all of which were originally anticipated. For example the transition dipole moments have been used to model electric field control molecular dynamics and Sysiphus cooling of molecules.