Double stars lose speed like a figure skater who slows their spin by extending their arms. In a close pair, one star transfers mass onto a white dwarf. The magnetic field of the donor star, consisting mostly of hydrogen, entangles the outflowing gas and flings it away, as if the skater's arms were pulling the spin outward. This magnetic braking reduces the system's orbital period. Previously, it was thought that when a star becomes fully convective (its interior churns like a boiling liquid), braking almost disappears. But analysis using spectroscopy showed: the weakening is only 2–3 times. That's enough to explain the observed periods of binary systems and the sizes of donor stars. Surprisingly, even with full convection, the magnetic field doesn't switch off — the skater slows down smoothly, not abruptly. This picture refines the evolution of stellar pairs and the nature of the magnetic dynamo, which also operates in our Sun, which once spun many times faster.
as written by the authors arXiv:2603.14560
Context. For decades, reproducing the orbital period distribution of non-magnetic Cataclysmic Variables (CVs) seemed to require a drastic decrease, usually termed disruption, of angular momentum loss through magnetic braking at the fully convective boundary, which argued for a change in the dynamo mechanism operating in fully and partially convective stars. However, recent studies showed that the magnetic braking prescription traditionally used in CV evolution theory is clearly outdated as saturation, that is, a weak period dependence for rapidly rotating stars, is not included. Aims. Here we test an updated version of a saturated magnetic braking prescription that has been developed to explain the spin-down of single stars in the context of CV evolution. This prescription contains a boosting and a disruption parameter that represent the change in the strength of magnetic braking at the fully convective boundary. Methods. We performed state of the art MESA simulations for CVs with the revised saturated magnetic braking prescription. Results. As in previous studies, we found that magnetic braking needs to be stronger in close binaries than in single stars and that, in contrast to what is observed in single stars, magnetic braking needs to be reduced at the fully convective boundary. However, in contrast to previous studies of CV evolution, only a moderate disruption by a factor of 2 - 3 is sufficient to explain key features of the CV orbital period distribution and the measured mass-radius relation for CV donors. Conclusions. The relatively small decrease of the efficiency of magnetic braking at the fully convective boundary might have implications for our understanding of dynamo models for fully and partially convective stars.
The same magnetic braking mechanism works for single stars too: our Sun in its youth rotated several times faster, and now it takes almost a month to complete one turn.