MAGNETIC-FIELD MODELS FOR A-STARS AND B-STARS V - THE MAGNETIC-FIELD AND PHOTOMETRIC VARIATION OF 84 URSAE MAJORIS

We present new longitudinal magnetic field and Stromgren photometric m easurements(1) of the Bp star 84 UMa. From these data we determine a n ew rotational period P-rot = 1.(d)38576 +/- 0.(d)00080, which is incon sistent with previously published values. The magnetic measurements in dicate the presence of a weak, dominantly dipolar magnetic field in th e photosphere of 84 UMa. We employ a new value of the rotational axis inclination in conjunction with the magnetic data in order to constrai n the magnetic field geometry. We derive the following dipole oblique rotator parameters: i = 59(-9)(+17)degrees, beta = 48(-29)(+17)degrees , and B-d = 1620(-30)(+1270) G. A precise value of the radius (which w e calculate in conjunction with the inclination using the Hipparcos pa rallax) 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 s uggestion by Hubrig & Mathys (1994) that the magnetic Ap stars may be near the end of their main sequence life.