Mechanisms for magnetic braking boost and disruption: The role of irradiation-driven winds and the convective turnover time spike in cataclysmic variables

¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, PR China
² Kazan Federal University, Kremlevskaya Str.18, 420008 Kazan, Russia

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 March 2026
Accepted: 13 May 2026

Abstract

Context. The saturated, boosted, and disrupted (SBD) magnetic braking (MB) model is an empirical prescription that has recently gained support from observations of diverse close binary systems. Different boosting (K) and disruption (η) parameters appear necessary for different systems, but their physical origins remain uncertain.

Aims. We aim to identify the physical mechanisms that boost MB and cause its disruption at the fully convective boundary in cataclysmic variables (CVs).

Methods. We modelled CV evolution using the MESA code and compared the results with observed CV properties. We computed the convective turnover time (τ_c) directly from the donor’s structure rather than adopting empirical relations. We also included irradiation from the accreting white dwarf, which heats the donor’s outer layers and can drive additional winds that enhance MB.

Results. The structure-based τ_c calculation reveals a pronounced spike as the donor approaches full convection, which drives the disruption parameter (η) and initiates the period gap in CVs. The outcome of irradiation is sensitive to the accretion, irradiation, and wind efficiencies, as well as to the base wind mass-loss rate of M-dwarf donors, all of which are poorly constrained from observations. Despite these uncertainties, plausible parameter choices allow irradiation-driven winds to provide the required boost (K) during accreting phases. We refer to the combined prescription as the iτSBD MB model and find that it yields evolutionary tracks broadly consistent with the main CV properties.

Conclusions. Our iτSBD MB framework offers a physically motivated interpretation of the empirical boost and disruption factors in SBD MB for CV evolution. We suggest that the convective turnover time spike at the fully convective boundary may be the universal driver of MB disruption for fast-rotating stars in the saturated regime, while irradiation-driven winds may be the dominant mechanism boosting MB in accreting binaries and other strongly irradiated close systems.