Extremely high circular polarization of an Ursae Majoris

Abstract

Three-color polarimetric measurements of the novalike binary system AN UMa are reported. It is found that the time-dependent linear polarization has peak values of 9% in the B spectral region and 7% in the U region, while the circular polarization varies with phase from 9% to 35% in the B region, 5% to 25% in the V region, and 12% to 31% in the U region. An average light curve is plotted which indicates marked changes in brightness from cycle to cycle and a scatter of about 0.2 magnitude that is probably due to intrinsic flickering. These results and previous observations are taken as evidence that the primary component is a white dwarf with a magnetic field of at least 300 MG phase-locked into synchronous rotation with the secondary and that the periodic polarization variations and photometric minima are produced by the rotation of this primary. Noting that AN UMa and AM Her both exhibit similar strong optical polarization, it is suggested that they represent a distinct type of object and be called 'polars'.