Discovery of a strong magnetic field in the rapidly rotating B2 Vn star HR 7355

M.E. Oksala$^{1,2}$, G.A. Wade$^2$, W.L.F. Marcolino$^{3,4}$, J. Grunhut$^{2,5}$, D. Bohlender$^{6}$, N. Manset$^{7}$, R.H.D. Townsend$^{8}$, and the MiMeS Collaboration

1-Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA;
2 - Department of Physics, Royal Military College of Canada, P.O. Box 17000, Station Forces, Kingston, Ontario, Canada;
3 - LAM-UMR 6110, CNRS & Univ. de Provence, 38 rue Fr'{e}d'{e}ric Joliot-Curie, F-13388 Marseille cedex 13, France;
4 - Observat`{o}rio Nacional, Rua Jos'{e} Cristino, 77. CEP 20921-400, S~{a}oCrist'{o}v~{a}o, Rio de Janeiro, Brazil;
5 - Department of Physics, Engineering Physics & Astronomy, Queen's University, Kingston, Ontario, Canada;
6 - National Research Council of Canada, Herzberg Institute of Astrophysics, 5071 W. Saanich Rd., Victoria, BC V9E 2E7, Canada;
7 - Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Hwy, Kamuela HI 96743, USA;
8 - Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling Hall, 475 N Charter Street, Madison, WI 53706, USA

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.

Reference: MNRAS Letter, in press
Status: Manuscript has been accepted


Comments: Also available through astro-ph: arXiv:1003.3626