2021A&A...650A.197H

Aims. The goal of this work is to study magnetic fields of the cool, eclipsing binary star UV Piscium (UV Psc). This system contains two active late-type stars, UV Psc A (G5V) and B (K3V). To obtain a complete picture, the properties of both global and local magnetic field structures are studied for both components.
Methods. High-resolution intensity and circular polarisation spectra, collected in 2016 with the ESPaDOnS spectropolarimeter at the CFHT, were used to analyse the magnetic field of UV Psc. To increase the signal-to-noise ratio, the multi-line technique of least-squares deconvolution (LSD) was used to obtain average StokesIV profiles. Then, a Zeeman-Doppler imaging (ZDI) code was employed to obtain the large-scale magnetic field topology and brightness distribution for both components of UV Psc.In addition, the small-scale magnetic fields, not visible to ZDI, were studied using the Zeeman intensification of FeI lines.
Results. The orbital and fundamental parameters of the system were revised based on the new radial velocity measurements. Maps of the surface magnetic field for both components of UV Psc were obtained, the large-scale magnetic fields feature strong toroidal and non-axisymetric components. UV Psc A and B have average global field strengths of 137 G and 88 G, respectively. The small-scale fields are notably stronger, with average strengths of 2.5 and 2.2kG, respectively. Only ∼5% of the total magnetic field strength is recovered with ZDI. Our results are in agreement with previous studies of partly-convective stars. Overall, UV Psc A has a stronger magnetic field compared to UV Psc B. Due to the eclipsing binary geometry, certain magnetic field features are not detectable using circular polarisation only. An investigation into theoretical linear polarisation profiles shows that they could be used to reveal antisymmetric components of the magnetic field. This result also has implications for the study of exoplanetary transit hosts. The successful use of Zeeman intensification shows the method's ability to extract information on magnetic fields for stars rotating significantly more rapidly than in previous studies.