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Astron. Astrophys. 335, 973-978 (1998)

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Magnetic field models for A and B stars

V. The magnetic field and photometric variation of 84 Ursae Majoris

G.A. Wade 1, G.M. Hill 2, 4, 5, S.J. Adelman 3, N. Manset 4 and P. Bastien 4

1 Physics & Astronomy Department, The University of Western Ontario, London, Ontario, N6A 3K7, Canada
2 Hertzberg Institute of Astrophysics, Dominion Astrophysical Observatory, 501 West Saanich Road, Victoria, B.C., V8X 4M6, Canada
3 Department of Physics, The Citadel, 171 Moultrie Street, Charlston, SC 29409, USA
4 Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, PQ, H3C 3J7, Canada,
5 McDonald Observatory, University of Texas at Austin, P.O. Box 1337, Fort Davis, TX 79734, USA

Received 29 December 1997 / Accepted 14 April 1998

Abstract

We present new longitudinal magnetic field and Strömgren photometric measurements 1 of the Bp star 84 UMa. From these data we determine a new rotational period [FORMULA], which is inconsistent with previously published values.

The magnetic measurements indicate the presence of a weak, dominantly dipolar magnetic field in the photosphere of 84 UMa. We employ a new value of the rotational axis inclination in conjunction with the magnetic data in order to constrain the magnetic field geometry. We derive the following dipole oblique rotator parameters: [FORMULA], [FORMULA], and [FORMULA] G.

A precise value of the radius (which we calculate in conjunction with the inclination using the Hipparcos parallax) allows us to locate 84 UMa on the radius-effective temperature plane. Using theoretical evolutionary tracks (Schaller et al. 1992), we obtain values for the mass, age and surface gravity. These results place 84 UMa quite close to the ZAMS, a result inconsistent with the suggestion by Hubrig & Mathys (1994) that the magnetic Ap stars may be near the end of their main sequence life.

Key words: stars: chemically peculiar – stars: individual: 84 UMa – stars: magnetic fields – polarization

Send offprint requests to: G.A. Wade, (gwade@astro.uwo.ca)

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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