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

TL;DR

This paper reports the discovery of a magnetic field in the B1/B2V star sigma Lupi, linking magnetic activity to stellar wind behavior and confirming the star's rotation period through spectropolarimetric observations.

Contribution

First detection of a magnetic field in sigma Lupi, establishing its magnetic nature and rotation period, and supporting the oblique magnetic rotator model for this star.

Findings

Detected a variable longitudinal magnetic field of about 100 G.

Confirmed the star's rotation period as approximately 3.0182 days.

Magnetic field variations align with the star's photometric period.

Abstract

The ultraviolet stellar wind lines of the photometrically periodic variable early B-type star sigma Lupi were found to behave very similarly to what has been observed in known magnetic B stars, although no periodicity could be determined. AAT spectropolarimetric measurements with SEMPOL were obtained. We detected a longitudinal magnetic field with varying strength and amplitude of about 100 G with error bars of typically 20 G. This type of variability supports an oblique magnetic rotator model. We fold the equivalent width of the 4 usable UV spectra in phase with the well-known photometric period of 3.019 days, which we identify with the rotation period of the star. The magnetic field variations are consistent with this period. Additional observations with ESPaDOnS attached to the CFHT strongly confirmed this discovery, and allowed to determine a precise magnetic period. Like in the other magnetic B stars the wind emission likely originates in the magnetic equatorial plane, with maximum emission occurring when a magnetic pole points towards the Earth. The 3.0182 d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field. No helium or other chemical peculiarity is known for this object.