Magnetic field topologies of the bright, weak-field Ap stars theta Aurigae and epsilon Ursae Majoris

Abstract

Context. The brightest magnetic chemically peculiar stars θ Aur and ε UMa were targeted by numerous studies of their photometric and spectroscopic variability. Detailed maps of chemical abundance spots were repeatedly derived for both stars. However, owing to the weakness of their surface magnetic fields, very little information on the magnetic field geometries of these stars is available. Aims. In this study we aim to determine detailed magnetic field topologies of θ Aur and ε UMa based on modern, high-resolution spectropolarimetric observations. Methods. Both targets were observed in all four Stokes parameters using the Narval and ESPaDOnS spectropolarimeters. A multiline technique of least-squares deconvolution was employed to detect polarisation signatures in spectral lines. These signatures were modelled with a Zeeman-Doppler imaging code. Results. We succeeded in detecting variable circular and linear polarisation signatures for θ Aur. Only circular polarisation was detected for ε UMa. We obtained new sets of high-precision longitudinal magnetic field measurements using mean circular polarisation metal line profiles as well as hydrogen line cores, which are consistent with historical data. Magnetic inversions revealed distorted dipolar geometries in both stars. The Fe and Cr abundance distributions, reconstructed simultaneously with magnetic mapping, do not show a clear correlation with the local magnetic field properties, with the exception of a relative element underabundance in the horizontal field regions along the magnetic equators. Conclusions. Our study provides the first ever detailed surface magnetic field maps for broad-line, weak-field chemically peculiar stars, showing that their field topologies are qualitatively similar to those found in stronger-field stars. The Fe and Cr chemical abundance maps reconstructed for θ Aur and ε UMa are at odds with the predictions of current theoretical atomic diffusion calculations.