A surprising analysis of material churned up from the depths of massive stars shows that the mixing processes in these hot, bright stars are much more complicated than thought. The results will be presented at the RAS National Astronomy Meeting held at Queen’s University Belfast on Wednesday 2nd April.
The study, led by astronomers from Queen’s University, used the FLAMES instrument on the Very Large Telescope (VLT) to decipher the spectra of light emitted by over 800 stars and estimate the chemical composition of the stars’ surfaces. This is the most extensive survey of massive stars ever undertaken.
Massive stars rotate at speeds of up to a million kilometres per hour and this rotation drives huge circulatory currents. Models predict that gas from the star’s core, containing nitrogen and other elements produced in fusion reactions, should be thrust up to the surface. In the study, the team used the concentration of nitrogen measured at the surface to analyse the efficiency of mixing in the star. They found that nearly half the stars did not have the levels of nitrogen predicted, indicating rotation is not the only factor driving mixing.
Ian Hunter, who led the study, said "Current models of star rotation could be compared to a food processor – as you turn up the speed, the mixing between the layers of a star becomes more thorough and more nitrogen should be visible at the surface. However, 20% of the stars we looked at were slow rotators that were rich in nitrogen and another 20% were fast rotators without much nitrogen. The food processor model doesn’t seem to be working, or at least can’t explain the whole picture."
The FLAMES instrument can observe 140 stars at the same time and take a detailed spectrum of each simultaneously. Professor Philip Dufton, of Queen’s University, commented, "We’ve known for nearly a hundred years that very massive stars spin fast but how this affects their behaviour is still quite a controversial subject. It’s only now, with this unique European instrument, that we can study enough stars to draw some conclusions."
The team believe that magnetic fields could explain the slow rotating, well-mixed group. Previous studies imply a link between fossil magnetic fields, intrinsic magnetic fields left over from the star formation stage, and nitrogen-rich stellar surfaces. However, the physical processes involved in this magnetically driven mixing are still unknown.
The rapidly rotating, nitrogen-poor stars would fit in with model predictions if they were in binary systems. However, the surveys do not appear to show any evidence of companions for stars in this group.
Professor Norbert Langer of the University of Utrecht commented, "There is a mystery here that we need to understand. We now have to look at what is missing in our models – possibly magnetic fields – or maybe there are more stars in double systems than we thought and we don’t fully understand how they interact. After 100 years of studying spinning stars we still can’t explain all we see."
The stars analysed are located in two nearby galaxies, the Large and Small Magellanic Clouds. The study was carried out over 100 hours of VLT time by a consortium comprising 20 scientists from five European countries.
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