The NGC 346 massive star census

Abstract

Previous analyses of two large spectroscopic surveys of early-type stars in the Large Magellanic Cloud (LMC) have found an excess of nitrogen enriched B-type targets with a ve sin i ≤ 40 km s−1 compared with the predictions of single star evolutionary models that incorporate rotational mixing. By contrast, the number of such targets with 40 <  ve sin i ≤ 80 km s−1 was consistent with such models. We have undertaken a similar analysis for 61 B-type targets which lie towards the young cluster, NGC 346 in the Small Magellanic Cloud (SMC). These again have projected rotational velocities, ve sin i ≤ 80 km s−1, are not classified as supergiants, and are apparently single. Approximately 65% of these SMC targets could have nitrogen enhancements of less than 0.3 dex, which is 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, ve sin i ≤ 40 km s−1, 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−1; these percentages are similar to those found previously for the two LMC samples. For all three surveys, the presence of undetected binaries and other uncertainties imply that these percentages might be underestimated and that it is indeed possible for all the single stars with current rotational velocities of less than 40 km s−1 to be nitrogen enriched. Two possible explanations incorporate the effects of the magnetic field, via either a stellar merger followed by magnetic braking or the evolution of a single star with a large magnetic field. Both mechanisms would appear to be 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 Galactic evolutionary models that incorporate magnetic fields is encouraging in terms of the amount of nitrogen enrichment and its presence in stars near the zero-age main sequence.