The incidence of magnetic cataclysmic variables can be explained by the late appearance of white dwarf magnetic fields

Abstract

Context. Assuming that white dwarf (WD) magnetic fields are generated by a crystallization- and rotation-driven dynamo, the impact of the late appearance of WD magnetic fields in cataclysmic variables (CVs) has been shown to potentially solve several long-standing problems of CV evolution. However, recent theoretical works show that the dynamo idea might not be viable and that the late appearance of WD magnetic fields might be an age effect rather than related to the cooling of the core of the WD. Aims. We investigated the impact of the late appearance of WD magnetic fields on CV evolution assuming that the fields appear at fixed WD ages. Methods. We performed CV population synthesis with the BSE code to determine the fractions of CVs that become magnetic at different evolutionary stages. These simulations were complemented with MESA tracks that take into account the transfer of spin angular momentum to the orbit which can cause a detached phase. Results. We find that the observed fraction of magnetic CVs as a function of orbital period is well reproduced by our simulations, and that in many CVs the WD should become magnetic close to the period minimum. The detached phase generated by the transfer of spin angular momentum is longest for period bouncers. Conclusions. Interpreting the late appearance of strong WD magnetic fields as a simple age effect naturally explains the relative numbers of magnetic CVs in observed samples. As many period bouncers might detach for several gigayears, the late appearance of WD magnetic fields at a fixed age and independent of the core temperature of the WD can significantly reduce the predicted number of accreting period bouncers.