The NGC 346 massive star census: Nitrogen abundances for apparently
  single, narrow lined, hydrogen core burning B-type stars
  Analyses of large spectroscopic surveys of early-type stars in the LMC have
found an excess of nitrogen enriched B-type targets with a vsini<40 km/s
compared with the predictions of single star evolutionary models incorporating
rotational mixing. By contrast the number of such targets with 40<vsini<80 km/s
were consistent with such models. We have undertaken a similar analysis for 61
B-type similar targets, towards the young cluster, NGC 346 in the Small
Magellanic Cloud (SMC). Approximately 65% could have nitrogen enhancements of
less than 0.3 dex, consistent with them having experienced only small amounts
of mixing due to their low rotational velocities. However as with the previous
LMC surveys, an excess of stars with low projected rotational velocities,
vsini<40 km/s and significant nitrogen enrichments is found. This is estimated
to be approximately 5% of the total population of apparently single B-type
stars or 40% of all stars with current rotational velocities of less than 40
km/s. For all three surveys, the presence of undetected binaries and other
uncertainties imply that these percentages might be underestimated and indeed
it is possible that all single stars with current rotational velocities of less
than 40 km/s are nitrogen enriched. Possible explanations incorporate the
effects of magnetic fields, via either a stellar merger followed by magnetic
breaking or the evolution of a single star with a large magnetic field. Both
mechanisms are compatible with the observed frequency of nitrogen-enriched
stars in the Magellanic Clouds. Differences in the properties of the
nitrogen-enriched stars compared with the remainder of the sample would be
consistent with the former mechanism. For the latter, a qualitative comparison
with evolutionary models incorporating magnetic fields is encouraging in terms
of the amount of nitrogen enrichment and its presence in stars near the ZAMS.
