B fields in OB stars (BOB): Detection of a strong magnetic field in the O9.7 V star HD54879

Abstract

The number of magnetic stars detected among massive stars is small; nevertheless, the role played by the magnetic field in stellar evolution cannot be disregarded. Links between line profile variability, enhancements/depletions of surface chemical abundances, and magnetic fields have been identified for low-mass B-stars, but for the O-type domain this is almost unexplored. Based on FORS2 and HARPS spectropolarimetric data, we present the first detection of a magnetic field in HD54879, a single slowly rotating O9.7 V star. Using two independent and different techniques we obtained the firm detection of a surface average longitudinal magnetic field with a maximum amplitude of about 600 G, in modulus. A quantitative spectroscopic analysis of the star with the stellar atmosphere code FASTWIND results in an effective temperature and a surface gravity of 33000$\pm 1000$ K and 4.0$\pm 0.1$ dex. The abundances of carbon, nitrogen, oxygen, silicon, and magnesium are found to be slightly lower than solar, but compatible within the errors. We investigate line-profile variability in HD54879 by complementing our spectra with spectroscopic data from other recent OB-star surveys. The photospheric lines remain constant in shape between 2009 and 2014, although H$\alpha$ shows a variable emission. The H$\alpha$ emission is too strong for a standard O9.7 V and is probably linked to the magnetic field and the presence of circumstellar material. Its normal chemical composition and the absence of photospheric line profile variations make HD54879 the most strongly magnetic, non-variable single O-star detected to date.

16 pages, accepted for publication in A&A

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Cited by in corpus (12)

• The MiMeS survey of magnetism in massive stars: Magnetic analysis of the O-type stars
• The Magnetic Early B-type Stars III: A main sequence magnetic, rotational, and magnetospheric biography
• On the Origin of Stochastic, Low-Frequency Photometric Variability in Massive Stars
• The MiMeS survey of Magnetism in Massive Stars: Magnetic properties of the O-type star population
• Testing the fossil field hypothesis: could strongly magnetised OB stars produce all known magnetars?
• X-ray diagnostics of massive star winds
• Mixing of materials in magnetised core-collapse supernova remnants
• A short and sudden increase of the magnetic field strength and the accompanying spectral variability in the O9.7V star HD54879
• The very slow rotation of the magnetic O9.7V star HD54879
• Detection of anomalous element distribution in the extremely slowly rotating magnetic O9.7 V star HD 54879
• No evidence of a sudden change of spectral appearance or magnetic field strength of the O9.7V star HD 54879
• A magnetic field detection in the massive O-type bright giant 63 Oph