HR 5907: discovery of the most rapidly rotating magnetic early B-type star by the MiMeS collaboration

Abstract

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 0.508 276[superscript +0.000 015][subscript −0.000 012] 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 [plus or minus] 1000 K, a projected rotational velocity of 290 [plus or minus] 10 km s[superscript −1], an equatorial radius of 3.1 [plus or minus] 0.2R[subscript circled dot], a stellar mass of 5.5 [plus or minus] 0.5 M[subscript circled dot], and an inclination angle of the rotation axis to our line-of-sight of 70 [plus or minus] 10[degree symbol]. 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[superscript +2][subscript −1][degree symbol]). 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 disc.