Nitrogen line spectroscopy of O-stars II. Surface nitrogen abundances for O-stars in the Large Magellanic Cloud

Context. Nitrogen is a key element for testing the impact of rotational mixing on evolutionary models of massive stars. Recent studies of the nitrogen surface abundance in B-type stars within the VLT-FLAMES survey of massive stars have challenged part of the corresponding predictions. To obtain a more complete picture of massive star evolution, and to allow for additional constraints, these studies need to be extended to O-stars. Aims. This is the second paper in a series aiming at the analysis of nitrogen abundances in O-type stars, to establish tighter constraints on the early evolution of massive stars. In this paper, we investigate the Niv.4058 emission line formation, provide nitrogen abundances for a substantial O-star sample in the Large Magellanic Cloud, and compare our (preliminary) findings with recent predictions from stellar evolutionary models. Methods. Stellar and wind parameters of our sample stars were determined by line profile fitting of hydrogen, helium and nitrogen lines, exploiting the corresponding ionization equilibria. Synthetic spectra were calculated by means of the NLTE atmosphere/ spectrum synthesis code fastwind, using a new nitrogen model atom. We derived nitrogen abundances for 20 O- and 5 B-stars by analyzing all nitrogen lines (from different ionization stages) present in the available optical spectra. Results. The dominating process responsible for emission at Niv.4058 in O-stars is the strong depopulation of the lower level of the transition, which increases as a function of. M. Unlike the Niii triplet emission, resonance lines do not play a role for typical mass-loss rates and below. We find (almost) no problem in fitting the nitrogen lines, in particular the f features. Only for some objects, where lines from Niii/N iv/N v are visible in parallel, we need to opt for a compromise solution. For five objects in the early B-/late O-star domain that have been previously analyzed by different methods and model atmospheres, we derive consistent nitrogen abundances. The bulk of our sample O-stars seems to be strongly nitrogen-enriched, and a clear correlation of nitrogen and helium enrichment is found. By comparing the nitrogen abundances as a function of v sin i (Hunter-plot) with tailored evolutionary calculations, we identify a considerable number of highly enriched objects at low rotation. Conclusions. Our findings seem to support the basic outcome of previous B-star studies within the VLT-FLAMES survey. Owing to the low initial abundance, the detection of strong nitrogen enrichment in the bulk of O-stars indicates that efficient mixing takes place already during the very early phases of stellar evolution of LMC O-stars. For tighter constraints, however, upcoming results from the VLT-FLAMES Tarantula survey need to be waited for, which will comprise a much higher number of O-stars that will be analyzed based on similar methods as presented here.