Stromgren v photometry of σ Lup

Magnetic early B-type stars are rare. Indirect indicators are needed to identify them before investing in time-intensive spectropolarimetric observations. 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. 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 100G with error bars of typically 20G, 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 3.02d, 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 helium-strong star, with an enhanced nitrogen abundance and an underabundance of carbon, and has a chemically spotted surface. Conclusions. sigma 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±0.00043d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field.

磁性的早期B型星相对罕见，在投入耗时精深的分光偏振观测之前，需要借助间接指标来识别它们。本研究采用B型星中磁场最强的间接指标——围绕约静止波长的对称性紫外（UV）恒星风线的周期性变化。我们的目标是识别可能的磁性候选星，使其成为后续分光偏振观测以寻找磁场的目标。通过UV风线变化，B1/B2V型星sigma Lupi显现为新的磁性候选星。利用SEMPOL进行了AAT分光偏振测量。通过最小二乘去卷积法，确定了磁场在可见表面上的纵向分量。sigma Lupi的UV线变化与已知磁性B型星中的情况相似，但无法确定周期性。我们检测到幅度约为100G的变动的纵向磁场，误差棒通常为20G，这支持了斜向磁旋转配置。UV线的等效宽度变化、磁性和光学线的变化与3.02d的测光周期一致，我们将其与星的旋转周期相对应。附加的ESPaDOnS（安装于CFHT）观测进一步确认了这一发现，并允许精确确定磁周期。分析表明，sigma Lupi是一颗富含氦的恒星，氮含量增加，碳含量偏低，并具有化学斑点表面。结论：sigma Lupi是一颗磁性斜向旋转星，且为富含氦的恒星。与其他磁性B型星类似，紫外风发射似乎起源于接近磁赤道面，当磁极指向地球时发射达到最大。3.01972±0.00043d的磁旋转周期与测光周期一致，最大亮度对应于最大磁场。