Evolution of cataclysmic variables under different magnetic braking prescriptions

TL;DR

This study probes how different magnetic-braking prescriptions drive the secular evolution of cataclysmic variables using MESA, focusing on orbital-period evolution, mass-transfer rates, and donor response. By implementing five MB formalisms—Skumanich, Matt12, RM12, intermediate, and convection-boosted—the authors show that the period gap emerges only under sufficiently strong pre-gap MB, while self-regulated braking yields weak angular-momentum loss and no clear gap. The intermediate MB prescription offers the best overall agreement with nonmagnetic CV observations, reproducing gap location, width, and donor properties; stronger MB produces detachment and gap, whereas weaker braking favors continuous mass transfer and no gap. The results stress the importance of pre-convective-envelope MB strength and call for incorporating magnetic-field regulation of donor structure and WD-field effects to predict period distributions across magnetic and nonmagnetic CV populations.

Abstract

Context. The evolution of cataclysmic variables (CVs) - interacting binaries where a low-mass donor transfers matter to a white dwarf via an accretion disk - is critically controlled by magnetic braking (MB). Significant uncertainties persist regarding how distinct MB formalisms influence CV evolutionary pathways. Aims. We performed systematic simulations of CV evolution under five MB prescriptions using the MESA code: the classical Skumanich law and the Matt, Reiners & Mohanty (RM12), intermediate, and convection-boosted formalisms. Primary objectives included investigating their impact on orbital period distributions, mass-transfer rates, donor star evolution, and period gap characteristics. Methods. Evolutionary sequences were computed across all MB frameworks. We analyzed their effects on key observables: orbital period evolution, accretion rates, and period gap morphology. Results. Magnetic braking prescription selection fundamentally determines whether CV systems develop the characteristic period gap. The intermediate prescription provides optimal consistency with observations of nonmagnetic CVs, simultaneously reproducing the gap location and donor properties. Strong braking models (e.g., Skumanich) produce clear detachment phases, while self-consistent regulation models (Matt12 and RM12) maintain weak angular momentum loss and fail to form a gap, making them more prone to magnetic CVs. Conclusions. The presence or absence of the period gap is primarily governed by the strength and behavior of MB before the donor becomes fully convective. Future studies must further incorporate the regulatory effects of magnetic fields on donor structure to accurately predict the period distribution characteristics of magnetic CVs.

Figures (13)

• Figure 1: Evolutional tracks of the Skumanich model: mass-transfer rate as a function of orbital period for different donor masses—0.6 $M_\odot$ (top left), 0.8 $M_\odot$ (top right), 1.0 $M_\odot$ (bottom left), and 1.2 $M_\odot$ (bottom right). Each panel shows the evolutionary tracks for three different WD masses: 0.6 $M_\odot$ (blue), 0.8 $M_\odot$ (red), and 1.0 $M_\odot$ (green). The purple pentagrams denote observational data obtained from Pala2020, Pala2022, and Sarkar2024, as well as the additional sources listed in Table A.1. All systems are initialized at an orbital period of 0.4 day.
• Figure 2: Same as Fig. \ref{['fig:sku_model']} but for the Matt12 model.
• Figure 3: Same as Fig. \ref{['fig:sku_model']} but for the RM12 model.
• Figure 4: Same as Fig. \ref{['fig:sku_model']} but for the intermediate model.
• Figure 5: Same as Fig. \ref{['fig:sku_model']} but for the boosted model.