Discovery of a magnetic field in the early B-type star sigma Lupi

Henrichs, H. F. and Kolenberg, K. and Plaggenborg, B. and Marsden, S. C. and Waite, I. A. and Landstreet, J. D. and Wade, G. A. and Grunhut, J. H. and Oksala, M. E. (2012) Discovery of a magnetic field in the early B-type star sigma Lupi. Astronomy and Astrophysics, 545. pp. 1-10. ISSN 0004-6361

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Context: Magnetic early B-type stars are rare. Indirect indicators are needed to identify them before investing in time-intensive spectropolarimetric observations.
Aims: We use the strongest indirect indicator of a magnetic field in B stars, which is periodic variability of ultraviolet (UV) stellar wind lines occurring symmetric about the approximate rest wavelength. Our aim is to identify probable magnetic candidates which would become targets for follow-up spectropolarimetry to search for a magnetic field.
Methods: From the UV wind line variability the B1/B2V star σ 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.
Results: The UV line variations of σ 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 equivalent width 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 σ Lupi is a helium-strong star, with an enhanced nitrogen abundance and an underabundance of carbon, and has a chemically spotted surface.
Conclusions: σ Lupi is a magnetic oblique rotator, and is a He-strong star. Like in other magnetic B stars the UV wind emission appears to originate close to the magnetic equatorial plane, with maximum emission occurring when a magnetic pole points towards the Earth. The 3.01972 ± 0.00043 d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field.

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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: Copyright ESO 2012. This publication is copyright. It may be reproduced in whole or in part for the purposes of study, research, or review, but is subject to the inclusion of an acknowledgment of the source. Article no. A119
Faculty / Department / School: Historic - Faculty of Sciences - Department of Biological and Physical Sciences
Date Deposited: 30 Jul 2014 02:09
Last Modified: 02 Mar 2017 03:56
Uncontrolled Keywords: early-type stars; magnetic field; winds; outflows rotation; starspots
Fields of Research : 02 Physical Sciences > 0201 Astronomical and Space Sciences > 020110 Stellar Astronomy and Planetary Systems
02 Physical Sciences > 0201 Astronomical and Space Sciences > 020109 Space and Solar Physics
02 Physical Sciences > 0201 Astronomical and Space Sciences > 020107 Mesospheric, Ionospheric and Magnetospheric Physics
Socio-Economic Objective: E Expanding Knowledge > 97 Expanding Knowledge > 970102 Expanding Knowledge in the Physical Sciences
Identification Number or DOI: 10.1051/0004-6361/201219632

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