Biogenic elements in stars harbouring planets

Several thousand exoplanets are now known. What type of planets that form will be dictated by their chemical composition, especially the carbon-to-oxygen ratio.While astronomers are not yet able to observe the light from exoplanets and thus determine their chemical composition directly, it is possible to infer it from measurements of their host stars. The carbon-to-oxygen ratio is of particular importance for the formation of planets since it determines their overall chemistry: a terrestrial planet with C/O<1 will have silicate-based rocks while for C/O>1 graphite-based minerals will dominate. On Earth and in the rest of the solar system, O is more abundant than C, as also expected throughout most of the Universe. Recently, it has been claimed that many stars harbouring planets have very peculiar elemental abundances, including many with C/O>1, which has resulted in a great number of works studying the possible properties of such planets. C and O are of course also vital elements for life as we know it – they are biogenic elements – which makes it even more important to accurately know their abundances.

Determining the C and O abundances of stars is however notoriously difficult, which leaves the possibility that the claims of C/O ratios greater than unity are spurious. Great care must be exercised both in the observations and in the analysis of the different types of atomic and molecular C and O absorption lines available in stellar spectra. We have recently obtained exquisite optical as well as infrared high-resolution spectra with the Keck (Hawaii) and TNG (Canary Islands) telescopes. Our new observations should resolve this crucial issue once and for all, especially with the state-of-the-art analysis methodology and tools available within our research group at The Australian National University.

We are seeking an ambitious student keen on working on a topic of major importance for exoplanetary research using frontline observations and analyses. You will be analysing the optical and infrared stellar spectra to determine the C and O abundances in a large sample of stars; additional Keck and TNG observations for the project will be obtained in late 2015/early 2016.