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Title: Discovery of a magnetic field in the early B-type star sigma Lupi
Authors: H. F. Henrichs, K. Kolenberg, B. Plaggenborg, S. C. Marsden, I. A. Waite, J. D. Landstreet, G. A. Wade, J. H. Grunhut, M. E. Oksala, the MiMeS Collaboration

Magnetic early B-type stars are rare. To identify them, we use the strongest indirect indicator of a magnetic field in B stars, which is periodic variability of UV stellar wind lines occurring symmetric about the approximate rest wavelength. Probable magnetic candidates are targets for follow-up spectropolarimetry to search for a magnetic field. From the UV wind line variability the B1/B2V star sigma Lupi emerged as a new magnetic candidate star. AAT spectropolarimetric measurements with SEMPOL were obtained. The longitudinal component of the magnetic field integrated over the visible surface of the star was determined with the Least-Squares Deconvolution method. The UV line variations of sigma Lupi are similar to what is known in magnetic B stars, but no periodicity could be determined. We detected a varying longitudinal magnetic field with amplitude of about 100 G with error bars of typically 20 G, which supports an oblique magnetic-rotator configuration. The EW variations of the UV lines, the magnetic and the optical-line variations are consistent with the photometric period of 3.02 d, which we identify with the rotation period of the star. Additional observations with ESPaDOnS attached to the CFHT confirmed this discovery, and allowed the determination of a precise magnetic period. Analysis revealed that sigma Lupi is a He-strong star, with an enhanced N abundance and an underabundance of C, and has a chemically spotted surface. Like in other magnetic B stars the UV wind emission appears to originate close to the magnetic equatorial plane, with maximum emission when a magnetic pole points towards the Earth. The 3.01972(43) d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field.

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Blobrana


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Posts: 131433
Date:
Permalink  
 

Title: Discovery of the magnetic field of the B1/B2V star sigma Lupi
Authors: H.F. Henrichs, K. Kolenberg, B. Plaggenborg, S.C. Marsden, I.A. Waite, J. Landstreet, J. Grunhut, M. Oksala, G. Wade, the MiMeS Collaboration

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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