The aim of the ExoMol project is to compute comprehensive spectroscopic line lists for hot molecules thought likely to occur in the atmospheres of exoplanets, brown dwarfs and cool stars. The list of such molecules is quite long but only those for four or more atoms require the use of DiRAC resources. Our calculations have been split between two computers located at Cambridge: Darwin, which was used to diagonalise small matrices and to compute transition intensities, and COSMOS which was essential for diagonalising larger matrices. We encountered some performance issues with COSMOS when performing large matrix diagonalisations. This problem was referred to SGI who have very recently supplied us with a new diagonaliser for COSMOS. Our initial tests with this new diagonaliser are very encouraging and suggest that our performance issues should largely be resolved. However we have yet to use diagonaliser in productions runs and the results discussed below did not employ it.
Work using DiRAC during 2013 focussed on four main molecules: methane (CH4), phosphine (PH3), formaldehyde (H2CO) and sulphur trioxide (SO3).Methane is a major absorber in hot Jupiter exoplanets, brown dwarfs and cool carbon stars. There is a huge demand for a reliable hot methane line list and there are several groups working towards this objective. This project was therefore given priority. Our main result is that we have generated a new methane line list, called 10to10, which contains just under 10 billion transitions. This line list is complete for wavelengths longer than 1 μm and temperatures up to 1500 K It is by some distance the most comprehensive line list available for methane (see Fig.1). It is currently being actively used by about a dozen groups worldwide to model methane in a variety of astronomical objects (and one group for studies of the earth’s atmosphere). Fig. 2 shows the spectrum of the brown dwarf spectrum of 2MASS J0559-1404 compared to our simulations.
Our tests on this line list show that for photometric studies of the K, H, J bands show that previously available line lists (a) agree well with 10to10 at 300 K for the K and H bands but significantly underestimate J-band absorption due to lack of experimental data in this region; and (b) seriously underestimate absorption by methane for all bands at temperatures above 1000 K. We have also completed initial, room temperature line lists for PH3 and SO3, and a full line list for hot H2CO.