Observational effects of magnetism in O stars: surface nitrogen abundances^⋆,⋆⋆

¹ LUPM – UMR 5299, CNRS & Université Montpellier II, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France
e-mail: fabrice.martins@univ-montp2.fr
² Dept. of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, Ontario K7K 7B4, Canada
³ IRAP – UMR 5277, CNRS & Université Paul Sabatier, 16 Avenue Edouard Belin, 31400 Toulouse, France
⁴ LAM – UMR 6110, CNRS & Université de Provence, rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁵ NASA/GSFC, Code 665, Greenbelt, MD 20771, USA

Received: 7 September 2011
Accepted: 9 December 2011

Abstract

Aims. We investigate the surface nitrogen content of the six magnetic O stars known to date as well as of the early B-type star τ Sco. We compare these abundances to predictions of evolutionary models to isolate the effects of magnetic field on the transport of elements in stellar interiors.

Methods. We conduct a quantitative spectroscopic analysis of the sample stars with state-of-the-art atmosphere models. We rely on high signal-to-noise ratio, high resolution optical spectra obtained with ESPADONS at CFHT and NARVAL at TBL. Atmosphere models and synthetic spectra are computed with the code CMFGEN. Values of N/H together with their uncertainties are determined and compared to predictions of evolutionary models.

Results. We find that the magnetic stars can be divided into two groups: one with stars displaying no N enrichment (one object); and one with stars most likely showing extra N enrichment (5 objects). For one star (Θ¹ Ori C) no robust conclusion can be drawn due to its young age. The star with no N enrichment is the one with the weakest magnetic field, possibly of dynamo origin. It might be a star having experienced strong magnetic braking under the condition of solid body rotation, but its rotational velocity is still relatively large. The five stars with high N content were probably slow rotators on the zero age main sequence, but they have surface N/H typical of normal O stars, indicating that the presence of a (probably fossil) magnetic field leads to extra enrichment. These stars may have a strong differential rotation inducing shear mixing. Our results should be viewed as a basis on which new theoretical simulations can rely to better understand the effect of magnetism on the evolution of massive stars.