Discovery of a strong magnetic field in the rapidly rotating B2Vn star HR 7355

Abstract

We report the detection of a strong, organized magnetic field in the helium-variable early B-type star HR 7355 using spectropolarimetric data obtained with ESPaDOnS on the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. HR 7355 is both the most rapidly rotating known main-sequence magnetic star and the most rapidly rotating helium-strong star, with $v\sin i$ = 300 $\pm$ 15 km s${}^{-1}$ and a rotational period of 0.5214404 $\pm$ 0.0000006 days. We have modeled our eight longitudinal magnetic field measurements assuming an oblique dipole magnetic field. Constraining the inclination of the rotation axis to be between $38^{\circ }$ and $86^{\circ }$, we find the magnetic obliquity angle to be between $30^{\circ }$ and $85^{\circ }$, and the polar strength of the magnetic field at the stellar surface to be between 13-17 kG. The photometric light curve constructed from HIPPARCOS archival data and new CTIO measurements shows two minima separated by 0.5 in rotational phase and occurring 0.25 cycles before/after the magnetic extrema. This photometric behavior coupled with previously-reported variable emission of the H$\alpha$ line (which we confirm) strongly supports the proposal that HR 7355 harbors a structured magnetosphere similar to that in the prototypical helium-strong star, $\sigma$ Ori E.

6 pages, 3 figures. Accepted for publication in MNRAS Letters

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• Stellar Magnetism
• Discovery of a strong magnetic field in the rapidly rotating B2Vn star HR 7355
• The rapid magnetic rotator HR7355