Sodium content predicts the advanced evolution of globular cluster stars
Sen— New observations of a huge star cluster, made using the European Southern Observatory’s Very Large Telescope (VLT), have shown that the way in which stars evolve and end their lives depends on the amount of sodium they contain.
Detailed computer models predict that stars of a similar mass to the Sun would have a period towards the ends of their lives, called the asymptotic giant branch (AGB), when they undergo a final burst of nuclear burning and puff off a lot of their mass in the form of gas and dust.
For a short period of time this ejected material is lit up by the strong ultraviolet radiation from the star and creates a planetary nebula, before forming the next generations of stars. This cycle of rebirth is vital to explain the evolving chemistry of the Universe and also provides the material required for the formation of planets, and even the ingredients for organic life.
Now, Australian stellar theory expert Simon Campbell of the Monash University Centre for Astrophysics, Melbourne, has found tantalising suggestions that some stars may not follow the rules and might skip the AGB phase entirely.
"For a stellar modelling scientist this suggestion was crazy! All stars go through the AGB phase according to our models. I double-checked all the old studies but found that this had not been properly investigated. I decided to investigate myself, despite having little observational experience," he said.
Campbell and his team used ESO's VLT to study the light coming from stars in the globular star cluster NGC 6752 in the southern constellation of Pavo (The Peacock). This ball of ancient stars contains both a first generation of stars and a second that formed somewhat later. The two generations can be distinguished by the amount of sodium they contain.
The globular star cluster NGC 6752 in the constellation of Pavo. Image credit ESO, IAU and Sky & Telescope.
"FLAMES, the multi-object high-resolution spectrograph on the VLT, was the only instrument that could allow us to get really high-quality data for 130 stars at a time. And it allowed us to observe a large part of the globular cluster in one go," said Campbell.
Although the stars in a globular cluster all formed at about the same time, they usually contain two or more populations of stars with different amounts of light chemical elements such as carbon, nitrogen and, crucially for this new study, sodium.
The results were a surprise, all of the AGB stars in the study were first generation stars with low levels of sodium and none of the higher-sodium second generation stars had become AGB stars at all. As many as 70% of the stars were not undergoing the final nuclear burning and mass-loss phase.
"It seems stars need to have a low-sodium “diet” to reach the AGB phase in their old age. This observation is important for several reasons. These stars are the brightest stars in globular clusters, so there will be 70% fewer of the brightest stars than theory predicts. It also means our computer models of stars are incomplete and must be fixed!" concludes Campbell.
It is thought that stars which skip the AGB phase will evolve directly into helium white dwarf stars and gradually cool down over many billions of years. The team expects that similar results will be found for other star clusters and further observations are planned.