Abstract
ABSTRACT We have developed a numerical magnetohydrodynamic (MHD) model of the propeller candidate star AE Aqr using axisymmetric MHD simulations. We suggest that AE Aqr is an intermediate polar-type star, where the magnetic field is relatively weak and an accretion disc may form around the white dwarf. The star is in the propeller regime, and many of its observational properties are determined by the disc–magnetosphere interaction. Comparisons of the characteristics of the observed versus modelled AE Aqr star show that the model can explain many observational properties of AE Aqr. In a representative model, the magnetic field of the star is B ≈ 3.3 × 105 G and the time-averaged accretion rate in the disc is 5.5 × 1016 g s−1. Most of this matter is ejected into conically shaped winds. The numerical model explains the rapid spin-down of AE Aqr through the outflow of angular momentum from the surface of the star to the wind, corona, and disc. The energy budget in the outflows, 9 × 1033 erg s−1, is sufficient for explaining the observed flaring radiation in different wavebands. The time-scale of ejections into the wind matches the short time-scale variability in the light curves of AE Aqr.
Highlights
AE Aqr is a nova-like cataclysmic variable (CV)
AE Aqr could be classified as a typical intermediate polar (DQ Her) type CV, except that it has a number of unusual properties: (1) It shows flaring radiation in optical, ultraviolet and X-ray bands, which are all correlated with each other (e.g., Patterson 1979; Mauche et al 2012); (2) Flaring radiation in the radio band shows a non-thermal spectrum typical for electrons radiating in a magnetized plasma (Bastian et al 1988)
The Doppler tomograms based on the analysis of these lines 1 are different from the tomograms of other intermediate polars, which typically indicate the presence of an accretion disc (e.g., Marsh et al 1990)
Summary
AE Aqr is a nova-like cataclysmic variable (CV). It consists of a magnetic white dwarf and a late-type companion star with a spectral type of K3-K5. Wang & Robertson (1985) studied the propeller regime in two-dimensional numerical simulations using polar coordinates, and studied the processes in the equatorial plane They observed that the matter of the inner disc interacts with the magnetosphere of the star due to the magnetic interchange instability (see Arons & Lea 1976). The propeller regime has been studied in axisymmetric simulations (Ustyugova et al 2006; Romanova et al 2005, 2009, 2018; Lii et al 2014) These simulations have shown that the disc-magnetosphere interaction is a strongly non-stationary process, where the inner disc oscillates and most of the matter is ejected into the outflows from the discmagnetosphere boundary, while a smaller amount of matter accretes onto the star. In Appendix A, we provide the details of our numerical model
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