We report the detection of a strong, reversing magnetic field and variable H-alpha emission in the bright helium-weak star HD 176582 (HR 7185). Spectrum, magnetic and photometric variability of the star are all consistent with a precisely determined period of 1.5819840 +/- 0.0000030 days which we assume to be the rotation period of the star. From the magnetic field curve, and assuming a simple dipolar field geometry, we derive a polar field strength of approximately 7 kG and a lower limit of 52 degrees for the inclination of the rotation axis. However, based on the behaviour of the H-alpha emission we adopt a large inclination angle of 85 degrees and this leads to a large magnetic obliquity of 77 degrees. The H-alpha emission arises from two distinct regions located at the intersections of the magnetic and rotation equators and which corotate with the star at a distance of about 3.5 R* above its surface. We estimate that the emitting regions have radial and meridional sizes on the order of 2 R* and azimuthal extents (perpendicular to the magnetic equator) of less than approximately 0.6 R*. HD 176582 therefore appears to show many of the cool magnetospheric phenomena as that displayed by other magnetic helium-weak and helium-strong stars such as the prototypical helium-strong star sigma Ori E. The observations are consistent with current models of magnetically confined winds and rigidly-rotating magnetospheres for magnetic Bp stars.
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