Circular polarimetry of suspect wind-accreting pre-polars II

Context. The origin of white dwarf magnetic fields is an open question. Furthermore, such fields play a vital role in the evolution of interacting binary stars. Here we present a study of white dwarf fields in so called pre-polars (or low accretion rate polars, LARPs), i.e. magnetic cataclysmic variables (CVs), where the mass-losing secondary star is not in Roche lobe contact, but the systems experience accretion via stellar wind. Such systems play a crucial role in understanding the magnetic braking and the angular momentum evolution of close binaries.

Aims. We aim to identify a set of wind-accreting pre-polars amongst our sample of nine candidate systems. We also attempt to measure the white dwarf magnetic fields and study the accretion geometry associated with wind-accreting pre-polars.

Methods. We obtained optical circular imaging photopolarimetry and circular spectropolarimetry of the targets. These are used to model the cyclotron emission and to estimate the white dwarf magnetic field strengths.

Results. We find that in at least four out of our nine candidate systems, we can confirm the magnetic nature of the white dwarf. Of these, one system, ZTF J1737+4013, appears to be an eclipsing polar. Furthermore, ZTF J2220+0721 and ZTF J2353+4153 appear to be very strong candidates for wind-accreting pre-polars (or LARPs), with the fourth, ZTF J0056+4926, showing strong variable Hα emission, atypical of pre-polars. However, the emission could be related to the strong activity of the donor star dominating the optical spectrum. One system, ZTF J0504+2145, is likely a nova-like CV. The exact nature of the other systems remains unclear.