EXOMOL: MOLECULAR LINE LISTS FOR EXOPLANET AND OTHER ATMOSPHERES

Author Institution: Department of Physics and Astronomy, University College London, London, WC1E 6BT, UK; Technische Universitat Dresden, Physikalische Chemie, D--01062 Dresden, GermanySpectral characterization of astrophysical objects cool enough to form molecules in their atmospheres (cool stars, extrosolar planets and planetary discs) requires considerable amount of fundamental molecular data. The existing molecular line lists (with some exceptions) are however not sufficiently accurate and complete. We present a new (five years) European Union project ExoMol aimed at bridging this gap: ExoMol will generate comprehensive line lists for \textit{all} molecules likely to be observable in exoplanet atmospheres in the foreseeable future. This is a huge undertaking which will mean providing in excess of 10$^{11}$ spectral lines for a large variety of molecular species. %Although the calculation of a rotation-vibration line list for molecules %with three atoms is becoming more routine, the issues involved in %calculating such lists for larger species are formidable. %This will also require developing new procedures, particularly to deal with %the larger molecules of interest which, up until now, have been considered %to be beyond the sort of detailed study anticipated here. The physics of molecular absorptions is complex and varies between different classes of absorbers. The project will therefore be divided into following topics (a) diatomic, (b) triatomics, (c) tetratomics, (d) methane and (e) larger molecules. Each of which will require special techniques will be required in each case. The majority of diatomic systems to be tackled are open shell species involving a transition metal atom; the opacity is provided by the transitions between the many low lying electronic states of the system. The calculation of rotation-vibration line lists for closed-shell triatomic systems is now relatively straightforward provided enough care is taken in deriving the potential energy surface. For H$_2$S calculations are in progress: the unusual properties of the dipole moment will also require careful treatment. Accurate rotation-vibration line lists for hot tetratomic molecules such as ammonia, acetylene, hydrogen peroxide and formaldehyde, are at the very limit of what is computationally possible at present. These will be computed either by direct calculation of the full line list, such as our hot ammonia line list BYTe developed recently [S.~N. Yurchenko, R.~J. Barber , and J.~Tennyson, \textit{Mon. Not. R. Astron. Soc., in press}, (2011)], or by use of high accuracy vibrational calculations onto which the rotational structure will be grafted using ideas developed from the standard perturbation theory approach to molecular spectroscopy. Tests for acetylene suggest that this latter approach, although intrinsically less accurate, is still able to give reliable results [A. Urru, I. N. Kozin, G. Mulas, B. J. Braams, and J. Tennyson, \textit{Mol. Phys.}, \textbf{108}, 1973 (2010)]. Data from this project can be accessed at www.exomol.com. %The molecules listed in Table~1 are not unique to exoplanets and the data %will be widely used for models of cool stars and brown dwarfs, as well %non-thermal processes such as masers and molecular fluorescence which is %observed in comets. %\mbox{}\\ %[1] S.~N. Yurchenko, R.~J. Barber , and J.~Tennyson, \textit{Mon. Not. R. %Astron. Soc., in press}, (2010). %[2] A. Urru, I. N. Kozin, G. Mulas, B. J. Braams, and J. Tennyson, %\textit{Mol. Phys.}, \textbf{108}, 1973 (2010). %Figure~1 presents results obtained with our recent (high temperature) %ammonia line list BYTe. The current database of laboratory data is partial %even at 296 K: besides missing all data at short wavelengths, there are %artificial windows in the data (eg at 4000 cm$^{-1}$). Our calculations are %comprehensive and accurate