The Magnetic Nature of the Cataclysmic Variable Period Gap

Abstract: One of the most important problems in the context of cataclysmic variables (CVs) is the lack of observations of systems with periods between 2 and 3.12 hours, known as the period gap. The orbital evolution of CVs with periods shorter than those in the gap is dominated by gravitational radiation while for periods exceeding those of the gap it is dominated by magnetic braking of the secondary star. Spruit & Ritter (1983) showed that as periods approach 3 hours and secondary stars become fully convective a sharp decline in magnetic dynamo and braking efficiency would result in such a gap. Recent X-ray observations finding coronal magnetic energy dissipation is similar in fully convective and partly radiative M dwarfs cast this theory into doubt. In this work, we use Zeeman-Doppler imaging observations culled from the literature to show that the complexity of the surface magnetic fields of rapidly rotating M dwarfs increases with decreasing rotation period. Garraffo et al. (2018) have shown that the efficiency of angular momentum loss of cool stars declines strongly with increasing complexity of their surface magnetic field. We explore the idea of Taam & Spruit (1989) that magnetic complexity might then explain the period gap. By generating synthetic CV populations using a schematic CV evolutionary approach, we show that the CV period gap can naturally arise as a consequence of a rise in secondary star magnetic complexity near the long period edge of the gap that renders a sharp decline in their angular momentum loss rate.