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Previous analyses of the spectra of OB-type stars in the Magellanic Cloudshave identified targets with low projected rotational velocities and relativelyhigh nitrogen abundances. The VFTS obtained spectroscopy for B-type 434 stars.We have estimated atmospheric parameters and nitrogen abundances using TLUSTYmodel atmospheres for 54 B-type targets that appear to be single, haveprojected rotational velocities <80 \kms and were not classified assupergiants. In addition, nitrogen abundances for 34 similar stars from aprevious survey have been re-evaluated. Approximately 75-80% of the targetshave nitrogen enhancements of less than 0.3 dex, consistent with them havingexperienced only small amounts of mixing. However, stars with low projectedrotational velocities <40 \kms and significant nitrogen enrichments are foundin both our samples and simulations imply that these cannot all be rapidlyrotating objects observed near pole-on. Adopting an enhancement threshold of0.6 dex, we observed five stars, yet stellar evolution models with rotationpredict only 1.25$\pm$1.11 for random stellar viewing inclinations. The excessof such objects is estimated to be 20-30% of all stars with current rotationalvelocities of less than 40 \kms and 2-4% of the total single B-type sample.These estimates constitute lower limits for stars that appear inconsistent withcurrent grids of stellar evolutionary models. Including targets with smallernitrogen enhancements of greater than 0.2 dex implies larger percentages oftargets that are inconsistent with current evolutionary models, viz. 70% of thestars with rotational velocities less than 40 \kms and 6-8% of the total singlestellar population. We consider possible explanations of which the mostpromising would appear to be breaking due to magnetic fields or stellar mergerswith subsequent magnetic braking.

astronomy and astrophysics

The VLT-FLAMES Tarantula Survey: XXVIII. Nitrogen abundances for  apparently single dwarf and giant B-type stars with small projected  rotational velocities