B fields in OB stars (BOB). Detection of a strong magnetic field in the non-peculiar O9.7V star HD 54879

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 FORS 2 and HARPS spectropolarimetric data, we present the first detection of a magnetic field in HD 54879, 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  results in an effective temperature and a surface gravity of 33 000 ± 1000 K and 4.0 ± 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 HD 54879 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 Hα shows a variable emission. The Hα 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 HD 54879 the most strongly magnetic, non-variable single O-star detected to date.

B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD −57° 3509

“…This is consistent with an evolutionary age (see Table 3) that compares reasonably well with the age of the parent open cluster NGC 3293, 10.7 Myr (Lotkin & Matkin 1994). There is also reasonably good agreement between the spectroscopic distance of CPD −57 • 3509 (d spec = 2630 ± 370 pc) and the cluster distance of ∼2460 pc (Briquet et al 2013;Petit et al 2013;Alecian et al 2014;Neiner et al 2014;Fossati et al 2014Fossati et al , 2015aSikora et al 2015;Castro et al 2015, grey circles), and CPD −57 • 3509 (black dot). Error bars are 1σ uncertainties.…”

B fields in OB stars (BOB): Low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars

“…by Castro et al (2015). Figure 6 shows the outcome of the Bonn pipeline indicating the clear detection of the magnetic field at the ∼9σ level, already reported by Castro et al (2015).…”

“…Figure 6 shows the outcome of the Bonn pipeline indicating the clear detection of the magnetic field at the ∼9σ level, already reported by Castro et al (2015). The stellar photospheric spectrum does not present any morphological peculiarity, typical for example of Of?p stars, and its analysis did not reveal any chemical peculiarity.…”

“…For the chemically normal O9.7V star HD 54879 we detected a longitudinal magnetic field with a maximum strength of about 1 kG (see Castro et al 2015, for more details). HD 164492C is a massive binary system in the centre of the Trifid nebula for which we detected a magnetic field of about 600 G, although it is unclear which of the stars composing this system is magnetic (see Hubrig et al 2014a, for more details).…”

“…The stellar photospheric spectrum does not present any morphological peculiarity, typical for example of Of?p stars, and its analysis did not reveal any chemical peculiarity. The only distinctive feature in the spectrum of HD 54879 is a prominent Hα emission that Castro et al (2015) attributed to circumstellar material, as the comparison of the Hα line profile with that of the star defining the O9.7V spectral type excludes the stellar wind as the cause of the emission.…”