Boron as tracer of rotational mixing in early-type massive stars

Supervision: Norbert Przybilla

The chemical element boron is easily destroyed in rather shallow layers of stellar envelopes by proton collisions. Rotational mixing in the course of the evolution of stars therefore depletes the surface abundance of boron. This occurs on a time scale much faster than for any other element  being subject to nuclear reactions, e.g. carbon and nitrogen (CNO-cycle), which take place in the stellar core. Hence, boron abundances provide a unique observational constraints for testing evolution models of
rotating stars.

In this work, boron abundances shall be determined for a sample ofmassive early B-type stars, which we already have studied extensively from optical spectra. Spectral lines of single and double-ionized boron on the other hand are present solely in the ultraviolet spectral range, accessible only from space. High-resolution UV spectra will therefore be extracted from the Hubble Space Telescope and the International Ultraviolet Explorer archives. The analysis will be based on existing stellar model
atmospheres and spectral analysis tools. In the last step, the observational data will be confronted with the predictions from state-of-the-art evolution models for rotating massive stars from the Geneva group.