A Lifetime of Influence: Abstracts, Talks, and Posters
 

Rotational mixing in Magellanic Cloud B stars: Theory and Observation

Ines Brott
Sterrenkundig Instituut Utrecht, The Netherlands

Ian Hunter
Queen's University Belfast, Northern Ireland

Norbert Langer
Sterrenkundig Instituut Utrecht, The Netherlands

Danny Lennon
Space Telescope Institute, Baltimore, USA

Philip Duftion
Queen's University Belfast, Northern Ireland

Rotational mixing is potentially one of the most important processes in massive stars; its effectiveness still remains to be proven. While the VLT FLAMES Survey of O and B Stars undertook a major step in this direction, its results are ambiguous and raise important new questions.

Nitrogen is an easily observed tracer element for rotational mixing. It is produced in the stellar center and can be mixed gradually to the surface over the main sequence lifetime. So, in fast rotating stars one expects N enhancement towards the end of the main sequence, while slow rotators should not show any N enhancement.

We used the VLT FLAMES data to constrain the uncertain physics of rotational mixing in our stellar evolution code. However, when we simulate a population of single stars we find that there are two groups of stars that cannot be explained: (1) a group of fast rotating stars which do not show any evidence for rotational mixing and (2) a group of slow rotators with strong N enrichment.Clearly binary effects and strong magnetic fields have to be considered to explain those two groups.

The element boron can be used to distinguish between the rotational mixing or binary scenarios. Since boron can only exist in the coolest outer layers, the element will be gradually destroyed by rotational mixing, while in a mass transfer scenario practically Boron free material is dumped on the mass gainer.

Our single star population simulations quantify the expected amount of boron in fast and slow rotators and allow a comparison with measured boron abundances in galactic B-Stars.