Nitrogen chronology of massive main sequence stars

TL;DR

This paper introduces a new method using nitrogen surface abundance to estimate the age and inclination of massive main sequence stars, validated with observations from the LMC and SMC clusters.

Contribution

The paper develops a novel nitrogen chronology technique based on stellar evolution models to determine ages and inclination angles of massive stars.

Findings

Validated the method with 7 stars showing consistent nitrogen and isochrone ages.

Flagged 10 stars as non-rotationally mixed based on nitrogen and age discrepancies.

Provided constraints on inclination angles for 62 stars, revealing nitrogen abundance trends.

Abstract

Rotational mixing in massive main sequence stars is predicted to monotonically increase their surface nitrogen abundance with time. We use this effect to design a method for constraining the age and the inclination angle of massive main sequence stars, given their observed luminosity, effective temperature, projected rotational velocity and surface nitrogen abundance. This method relies on stellar evolution models for different metallicities, masses and rotation rates. We use the population synthesis code STARMAKER to show the range of applicability of our method. We apply this method to 79 early B-type main sequence stars near the LMC clusters NGC 2004 and N 11 and the SMC clusters NGC 330 and NGC 346. From all stars within the sample, 17 were found to be suitable for an age analysis. For ten of them, which are rapidly rotating stars without a strong nitrogen enhancement, it has been previously concluded that they did not evolve as rotationally mixed single stars. This is confirmed by our analysis, which flags the age of these objects as highly discrepant with their isochrone ages. For the other seven stars, their nitrogen and isochrone ages are found to agree within error bars, what validates our method. Constraints on the inclination angle have been derived for the other 62 stars,with the implication that the nitrogen abundances of the SMC stars, for which mostly only upper limits are known, fall on average significantly below those limits. Nitrogen chronology is found to be a new useful tool for testing stellar evolution and to constrain fundamental properties of massive main sequence stars. A web version of this tool is provided.