The Role of White Dwarfs in Cataclysmic Variable Spin-down

TL;DR

This study uses 3D magnetohydrodynamic simulations to analyze how white dwarfs influence the spin-down and wind accretion processes in cataclysmic variable systems, revealing moderate changes in angular momentum loss and variable accretion rates.

Contribution

It introduces a detailed 3D MHD model of white dwarf effects on system spin-down and wind accretion, showing these effects are significant but do not require major revisions of existing models.

Findings

Angular momentum loss rates vary by factors of 2-3 due to white dwarf effects.

Wind accretion rates depend on magnetic field alignment and orbital separation.

Efficient accretion can rapidly activate with decreasing orbital separation.

Abstract

We study the effect of a white dwarf on the spin-down of a cataclysmic variable system using a three-dimensional magnetohydrodynamic numerical model. The model includes the stellar corona, the stellar wind, and the WD mass and magnetic field. The existence of the WD modifies the system spin-down by physically blocking the stellar wind, restructuring the wind, channeling the wind towards the WD surface, and by modifying the shape and size of the Alfv\'en surface. The combination of these processes differs among a set of simple test cases, and the resulting angular momentum loss rates vary by factors of 2-3, and by factors of two relative to a test model with a single M dwarf. While the model employs some simplifications, the results suggest angular momentum loss schemes currently employed in cataclysmic variable studies do not require drastic revision. Insights are also gained on wind accretion. We find that efficient accretion switches on quite rapidly with decreasing orbital separation. Accretion rates depend on magnetic field alignment and should be modulated by magnetic cycles on the M dwarf. For particular values of white dwarf magnetic field strength, an efficient syphoning of coronal plasma from the inward facing M dwarf hemisphere occurs. Wind accretion rates are expected to vary by factors of 10 or more between fairly similar close binaries, depending on magnetic field strengths and orbital separation.