The MiMeS survey of magnetism in massive stars: CNO surface abundances of Galactic O stars^⋆

¹ LUPM, Université Montpellier 2, CNRS, Place Eugène Bataillon, 34095 Montpellier, France
e-mail: fabrice.martins@univ-montp2.fr
² LAM–UMR 6110, CNRS & Université de Provence, rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
³ Observatorio do Valongo, Universidade Federal do Rio de Janeiro, Ladeira Pedro António, 43, 20080-090 Rio de Janeiro, Brasil
⁴ Dept. of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, Ontario K7K 7B4, Canada
⁵ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
⁶ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble, UMR 5274, 38041 Grenoble, France
⁷ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁸ Dept. of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA

Received: 17 October 2014
Accepted: 14 November 2014

Abstract

Context. The evolution of massive stars is still partly unconstrained. Mass, metallicity, mass loss, and rotation are the main drivers of stellar evolution. Binarity and the magnetic field may also significantly affect the fate of massive stars.

Aims. Our goal is to investigate the evolution of single O stars in the Galaxy.

Methods. For that, we used a sample of 74 objects comprising all luminosity classes and spectral types from O4 to O9.7. We relied on optical spectroscopy obtained in the context of the MiMeS survey of massive stars. We performed spectral modelling with the code CMFGEN. We determined the surface properties of the sample stars, with special emphasis on abundances of carbon, nitrogen, and oxygen.

Results. Most of our sample stars have initial masses in the range of 20 to 50 M_⊙. We show that nitrogen is more enriched and carbon and oxygen are more depleted in supergiants than in dwarfs, with giants showing intermediate degrees of mixing. CNO abundances are observed in the range of values predicted by nucleosynthesis through the CNO cycle. More massive stars, within a given luminosity class, appear to be more chemically enriched than lower mass stars. We compare our results with predictions of three types of evolutionary models and show that for two sets of models, 80% of our sample can be explained by stellar evolution including rotation. The effect of magnetism on surface abundances is unconstrained.

Conclusions. Our study indicates that in the 20−50 M_⊙ mass range, the surface chemical abundances of most single O stars in the Galaxy are fairly well accounted for by stellar evolution of rotating stars.