The magnetic field geometry and rotational period of the Bp star HD 200311

We present a model of the magnetic field geometry of the Bp star HD 200311 based upon new mean longitudinal magnetic field measurements and previously published mean magnetic field modulus measurements by Mathys ct al. This is the fourth in a series of papers aimed at constructing magnetic field models for peculiar stars throughout the F0-B2 temperature range. The 21 longitudinal magnetic field measurements indicate the presence of a strong and variable line-of-sight magnetic field. These data do not define a smooth curve of variation when phased with the 26.0042-d period found by Adelman (1997) from Stromgren photometry. However, when phased with a period of 52.0084 d (twice Adelman's period). the longitudinal field data define a sinusoidal curve with extrema of -1.8 and + 1.8 kG. The mean magnetic field modulus measurements describe an approximately sinusoidal curve when phased with this same period. Adelman's photometry, when phased with this period, describes a double-wave variation with approximately equal maxima, one of which occurs close in phase to the longitudinal field maximum. This is consistent with an inhomogeneous chemical abundance distribution that is symmetric about the magnetic field axis, and similar in both magnetic hemispheres. The phased magnetic field data have been modelled assuming an oblique decentred magnetic dipole. The best-fitting oblique rotator parameters for this model are rotational asis inclination i = 28 degrees +/- 8 degrees or 90 degrees +/- 8 degrees, magnetic axis obliquity angle beta = 90 degrees +/- 8 degrees or 28 degrees +/- 8 degrees, polar strength of the magnetic dipole (when centred)B-d = -12.8 +/- 1 kG, and dipole decentring parameter a = 0.09 +/- 0.04.