Magnetic field models for A and B stars V. The magnetic field and photometric variation of 84 Ursae Majoris

We present new longitudinal magnetic field and Str ¨ omgren photometric measurements 1 of the Bp star 84 UMa. From these data we determine a new rotational period Prot = 1. d 38576±0. d 00080, which is inconsistent with previously pub- lished 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 inclina- tion in conjunction with the magnetic data in order to constrain the magnetic field geometry. We derive the following dipole oblique rotator parameters: i =5 9 +17 9 o , � =4 8 +17 29 o , and Bd = 1620 +1270 30 G. A precise value of the radius (which we calculate in conjunc- tion 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.