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arXiv · 1108.4556

Discovery of the magnetic field of the B1/B2V star \sigma Lupi

H. F. Henrichs·K. Kolenberg·B. Plaggenborg·S. C. Marsden·I. A. Waite·J. Landstreet·J. Grunhut·M. Oksala·G. Wade·the MiMeS Collaboration
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Abstract

In our search for new magnetic massive stars we use the strongest indirect indicator of a magnetic field in B stars, which is periodic variability of UV stellar wind lines occurring in a velocity range symmetric around zero. Our aim is to obtain follow-up spectropolarimetry to search for a magnetic field in magnetic candidate stars. We quantify UV wind line variability, and analyse its time behaviour. The B1/B2V star sigma Lup emerged as a new magnetic candidate star. AAT spectropolarimetric measurements with SEMPOL were obtained. The stellar wind line variations of sigma Lup are similar to what is known in magnetic B stars, but no periodicity could be determined. We detected a longitudinal magnetic field with varying strength and amplitude of about 100 G with error bars of typically 20 G, which supports an oblique magnetic-rotator configuration. The equivalent width variations of the UV lines, the magnetic and the optical line variations are consistent with the well-known photometric period of 3.02 days, which we identify with the rotation period of the star. Additional observations with ESPaDOnS at CFHT strongly confirmed this discovery, and allowed to determine a precise magnetic period. Further analysis revealed that $\sigma$ Lupi is a helium-strong star, with an enhanced nitrogen abundance and an underabundance of carbon, and has a spotted surface. We conclude that sigma Lup is a magnetic oblique rotator, and is a He-strong star. It is the 4th B star for which a magnetic field is discovered from studying only its wind variability. Like in the other magnetic B stars the wind emission originates in the magnetic equator, with maximum emission occurring when a magnetic pole points towards the Earth. The 3.02 d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field. A full paper will be submitted to A&A.

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H. F. Henrichs, K. Kolenberg, B. Plaggenborg, S. C. Marsden, I. A. Waite, J. Landstreet, J. Grunhut, M. Oksala, G. Wade, the MiMeS Collaboration. 2011-08-23. Discovery of the magnetic field of the B1/B2V star \sigma Lupi. https://doi.org/10.1063/1.3701907

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