Detection of a centrifugal magnetosphere in one of the most massive stars in the Oph star-forming cloud
Abstract
Recent XMM-Newton observations of the B2 type star rho Oph A indicated a periodicity of 1.205d, which was ascribed to rotational modulation. Since variability of X-ray emission in massive stars is frequently the signature of a magnetic field, we investigated whether the presence of a magnetic field can indeed be invoked to explain the observed X-ray peculiarity. Two FORS2 spectropolarimetric observations in different rotation phases revealed the presence of a negative (<Bz>all=-419+-101G) and positive (<Bz>all=538+-69G) longitudinal magnetic field, respectively. We estimate a lower limit for the dipole strength as Bd = 1.9+-0.2kG. Our calculations of the Kepler and Alfven radii imply the presence of a centrifugally supported, magnetically confined plasma around rho Oph A. The study of the spectral variability indicates a behaviour similar to that observed in typical magnetic early-type Bp stars.
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