* Astronomy

Title: HR 5907: Discovery of the most rapidly rotating magnetic B-type star by the MiMeS Collaboration
Authors: J.H. Grunhut, Th. Rivinius, G.A. Wade, R.H.D. Townsend, W.L.F. Marcolino, D.A. Bohlender, Th. Szeifert, V. Petit, J.M. Matthews, J.F. Rowe, A.F.J. Moffat, T. Kallinger, R. Kuschnig, B.D. Guenther, S.M. Rucinski, D. Sasselov, W.W. Weiss, the MiMeS Collaboration

We report the discovery and analysis of a very strong magnetic field in the rapidly rotating early B-type star HR 5907, based on observations obtained as part of the Magnetism in Massive Stars (MiMeS) project. We infer a rotation period of 0.508276 +0.000015/-0.000012 d from photometric and H{\alpha} EW measurements, making this the shortest period, non-degenerate, magnetic massive star known to date. From the comparison of IUE UV and optical spectroscopy with LTE BRUCE/KYLIE models we find a solid-angle integrated, uniform black-body temperature of 17 000 ±1000 K, a projected rotational velocity of 290 ±10 km/s, an equatorial radius of 3.1 ±0.2 solar radii, a stellar mass of 5.5 ±0.5 solar masses, and an inclination angle of the rotation axis to our line-of-sight of 70 ±10°. Our measurements of the longitudinal magnetic field, which vary between -500 and -2000 G, phase coherently with the rotation period and imply a surface dipole field strength of ~15.7 kG. On the other hand, from fits to mean Least-Squares Deconvolved Stokes V line profiles we infer a dipole field strength of ~10.4 kG. This disagreement may result from a magnetic configuration more complex than our model, and/or from the non-uniform helium surface abundance distribution. In either case we obtain a magnetic obliquity nearly aligned with the rotation axis ({\beta} = 7+2/-1°). Our optical spectroscopy also shows weak variability in carbon, silicon and nitrogen lines. The emission variability in hydrogen Balmer and Paschen lines indicates the presence of a dense, highly structured magnetosphere, interpreted as a centrifugally supported, magnetically confined circumstellar disk.

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Position (J2000):     R.A. 15 53 55.86404  |  Dec. -23° 58' 41.1522''