The effects of surface fossil magnetic fields on massive star evolution – III. The case of τ Sco

$\tau$ Sco, a well-studied magnetic B-type star in the Upper Sco association, has number of surprising characteristics. It rotates very slowly and shows nitrogen excess. Its surface field is much more complex than purely dipolar configuration which unusual for massive star. We employ CMFGEN radiative transfer code to determine fundamental parameters CNO helium abundances. Then, we MESA GENEC stellar evolution models accounting effects fields. To reconcile Sco's properties with single-star models, an increase necessary efficiency rotational mixing by factor 3 10 braking 10. The spin down could be explained assuming decay scenario. However, simultaneous chemical enrichment challenges Previous works indeed suggested merger origin Sco. scenario also faces similar as our explain slow rotation In conclusion, channel seems less likely versatile these discrepancies, while other potential binary-evolution channels still require further assessment whether they may self-consistently observables