Volume 545, September 2012
|Number of page(s)||10|
|Published online||18 September 2012|
Astronomical Institute “Anton Pannekoek”, University of
Science Park 904,
2 Department of Astrophysics/IMAPP Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
4 Instituut voor Sterrenkunde, K. U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
5 Australian Astronomical Observatory, PO Box 296, Epping, NSW 1710, Australia
6 Centre for Astronomy, School of Engineering and Physical Sciences, James Cook University, 4811 Townsville, Australia
7 Faculty of Sciences, University of Southern Queensland, Toowoomba, Qld 4350, Australia
8 Dept. of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada
9 Armagh Observatory, College Hill, Armagh, BT61 9DG, Northern Ireland
10 Dept. of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, Ontario, Canada
11 Dept. of Physics and Astronomy, University of Delaware, Newark, DE, USA
Received: 20 May 2012
Accepted: 19 July 2012
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.
Key words: stars: early-type / stars: magnetic field / stars: winds, outflows / stars: rotation / starspots / stars: abundances
Based on observations obtained by the International Ultraviolet Explorer, collected at NASA Goddard Space Flight Center and Villafranca Satellite Tracking Station of the European Space Agency. Optical observations have been obtained at the Anglo Australian Telescope (AAT) and at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.
The SMARTS 0.9 m photometry is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/545/A119
© ESO, 2012
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