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

TL;DR

This paper models magnetic braking in cataclysmic variables, proposing that a convective turnover time spike and irradiation-driven winds explain observed disruption and boosting phenomena, aligning with empirical models.

Contribution

It introduces the iτSBD magnetic braking model, combining structure-based convective turnover time calculations and irradiation effects to physically explain empirical boost and disruption factors.

Findings

Convective turnover time spike drives magnetic braking disruption at the fully convective boundary.

Irradiation-driven winds can plausibly provide the magnetic braking boost during accretion phases.

The model's evolutionary tracks match key properties of observed cataclysmic variables.

Abstract

The saturated, boosted, and disrupted magnetic braking (SBD MB) model is an empirical prescription that has recently gained support from close-binary observations. Different boosting ($K$) and disruption ($\eta$) parameters appear necessary for different systems, but their physical origins remain uncertain. We aim to identify the mechanisms that boost magnetic braking (MB) and cause its disruption at the fully convective boundary in cataclysmic variables (CVs). We modelled CV evolution with MESA and compared the results with observed CV properties. We computed the convective turnover time ($\tau_c$) 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. The structure-based $\tau_c$ calculation reveals a pronounced spike as the donor approaches full convection, which drives the disruption parameter $\eta$ and initiates the period gap in CVs. The outcome of irradiation is sensitive to the accretion, irradiation, and wind efficiencies, 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 i$\tau$SBD MB model and find that it yields evolutionary tracks broadly consistent with the main CV properties. Our i$\tau$SBD 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 drive 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.