Analysis of new spectropolarimetric data of AR UMa

Optical spectropolarimetric observations of the high field AM Herculis system AR UMa during a low state of activity reveal a strongly structured circular polarization spectrum with a peak polarization of about 6 per cent. The strongest features can be identified with Zeeman split components of Hα and Hβ in an average magnetic field of 140-170 MG. Attempts at modelling the data using centred and off-centred dipolar fields have only been partially successful. Mildly off-centred dipoles appear to be in better overall agreement with observations, but only marginally. Our favoured models belonging to this class have a dipolar field strength B_d ∼ 200-235 MG displaced from the centre of the star by ∼5-15 per cent of the stellar radius. These models also present a reasonable fit to the photospheric Lyα π and σ⁺ components in the ultraviolet spectral region which are less sensitive to field structure. Our failure to obtain detailed agreement with the optical spectropolarimetry suggests that AR UMa, in common with some other well studied magnetic dwarfs, may have a field structure which differs from that of a dipole. Our study supports a previous suggestion that the variable emission feature near 1730 Å and the feature near 2220 Å, which are seen superimposed on the photospheric spectrum, are caused by cyclotron emission from a weakly accreting shock. We identify these features with cyclotron harmonics 4 and 3 respectively, in a field of 154 MG, which is the field strength expected near the weaker pole of our dipole model with B _d = 235 and an offset of 15 per cent of the stellar radius along the dipolar axis. A field of 154 MG can also explain the bump near 4450 Å present in the high state circular polarization data of AR UMa, as harmonic number 2.