Characterisation of the magnetic field of the Herbig Be star HD 200775 - AstrophysicsReport as inadecuate

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Abstract: After our recent discovery of four magnetic Herbig stars, we have decided tostudy in detail one of them, HD 200775, to determine if its magnetic topologyis similar to that of the main sequence magnetic stars. With this aim, wemonitored this star in Stokes I and V over more than two years, using the newspectropolarimeters ESPaDOnS at CFHT, and Narval at TBL. Using our data, wefind that HD 200775 is a double-lined spectroscopic binary system, whosesecondary seems similar, in temperature, to the primary. We determine theluminosity ratio of the system, and using the luminosity of the system found inliterature, we derive the luminosity of both stars. From our measurements ofthe radial velocities of both stars we determine the ephemeris and the orbitalparameters of the system. We have fitted 30 Stokes V profiles simultaneously,using a chi2 minimisation method, with a decentered-dipole model. The best-fitmodel provides a rotation period of 4.3281 d an inclination angle of 60degrees, and a magnetic obliquity angle of 125 degrees. The polar strength ofthe magnetic dipole field is 1000 G, which is decentered by 0.05 R* from thecenter of the star. The derived magnetic field model is qualitatively identicalto those commonly observed in the Ap-Bp stars, which bring strong argument infavour of the fossil field hypothesis, to explain the origin of the magneticfields in the main sequence Ap-Bp stars. Our determination of the inclinationof the rotation axis leads to a radius of the primary which is smaller thanthat derived from the HR diagram position. This can be explained by a largerintrinsic luminosity of the secondary relative to the primary, due to a largercircumstellar extinction of the secondary relative to the primary.

Author: E. Alecian RMC, C. Catala LESIA, G.A. Wade RMC, J.-F. Donati LATT, P. Petit LATT, J.D. Landstreet UWO, S. Bagnulo Armagh Observat



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