The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds

TL;DR

This study measures nitrogen abundances in Be-type stars in the Magellanic Clouds and compares them with stellar evolution models, revealing discrepancies that challenge current understanding of stellar rotation and chemical mixing.

Contribution

It provides the first detailed nitrogen abundance measurements for Be-type stars in the Magellanic Clouds and tests their consistency with evolutionary predictions.

Findings

Nitrogen abundances are lower than model predictions for near-critical rotation.

Observed rotational velocities do not match models assuming extensive mixing.

Results suggest revisions are needed in stellar evolution models for Be-type stars.

Abstract

Aims. We compare the predictions of evolutionary models for early-type stars with atmospheric parameters, projected rotational velocities and nitrogen abundances estimated for a sample of Be-type stars. Our targets are located in 4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11 region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346. Methods. Atmospheric parameters and photospheric abundances have been determined using the non-LTE atmosphere code tlusty. Effective temperature estimates were deduced using three different methodologies depending on the spectral features observed; in general they were found to yield consistent estimates. Gravities were deduced from Balmer line profiles and microturbulences from the Si iii spectrum. Additionally the contributions of continuum emission from circumstellar discs were estimated. Given its importance in constraining stellar evolutionary models, nitrogen abundances (or upper limits) were deduced for all the stars analysed. Results. Our nitrogen abundances are inconsistent with those predicted for targets spending most of their main sequence life rotating near to the critical velocity. This is consistent with the results we obtain from modelling the inferred rotational velocity distribution of our sample and of other investigators. We consider a number of possibilities to explain the nitrogen abundances and rotational velocities of our Be-type sample.