All elements other than hydrogen and helium are forged by nuclear reactions in the fiery interiors of stars. When stars die they eject some of their nuclear-processed material to the interstellar medium from which subsequent generations of stars are born, making the amount of elements heavier than helium in the Universe to steadily increase with time. Each element has its own characteristic origin in terms of what type of stars produced it and when. For example, the oxygen we breathe is thought to have been generated by stars with a mass more than eight times that of the Sun while the carbon in our bodies is believed to come from stars like our own Sun. Since solar-like stars live for many billions of years, these stars would not have had time to enrich their surroundings with carbon during the earliest cosmic epochs when the Milky Way Galaxy was formed. The expectation would then be that the astronomical fossils from this period -- the oldest stars in the Galaxy surviving until now -- should contain relatively little carbon.

Astronomers can determine the chemical composition of stars by studying the radiation they emit with different elements producing absorption lines at distinct wavelengths in the stellar spectrum. Coupled with a realistic model for how the spectrum is generated in the outer layers of stars, the stellar atmospheres, one can then infer how much say carbon or iron a star contains. A team led by PhD student Damian Fabbian from Australian National University and Prof. Martin Asplund from Max Planck Institute for Astrophysics has studied some of the oldest stars known in the Milky Way using observations obtained with European Southern Observatory's Very Large Telescope in Chile. In addition they have modelled the stellar spectrum formation taking into account atomic processes not previously considered, which makes the analysis more reliable.

To their surprise, the team members found that these very old stars contained large amounts of carbon while at the same time less oxygen than previous studies had claimed. In fact, the measured ratio of the carbon abundance to the oxygen abundance in the most chemically pristine of the stars studied is the same as for the Sun. The observed carbon and oxygen atoms in the stars were produced in a previous generation of stars, the very first stars born after the Big Bang. These so-called first stars have never been observed directly as they have now all died, but they must have been very different from the stars existing today. Indeed, recent theoretical calculations of the evolution of such stars imply that even very massive stars containing initially only hydrogen and helium can produce the large amounts of carbon observed. The new observations may therefore reveal the tell-tale nuclear signature of the elusive first stars.

Martin Asplund

Publications

Damian Fabbian, Martin Asplund, Paul Barklem, Mats Carlsson and Dan Kiselman, "Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity", 2008, submitted to Astronomy & Astrophysics

Damian Fabbian, Poul Erik Nissen, Martin Asplund and Max Pettini, "The [C/O] ratio at low metallicity: constraints on early chemical evolution from observations of Galactic halo stars", 2008, submitted to Astronomy & Astrophysics

Damian Fabbian, "Chemical compositions of stars in the light of non-LTE spectral line formation: the evolution of carbon and oxygen in the Galaxy", 2008, PhD thesis at the Australian National University