Polar (cataclysmic variable)

A Polar (also AM Herculis Star) is a highly magnetic type of cataclysmic variable binary star system. Like other cataclysmic variables (CVs), polars contain two stars: an accreting white dwarf (WD), and a low-mass donor star (usually a red dwarf) which is transferring mass to the WD as a result of the WD's gravitational pull. (In technical terms, the donor star overfills its Roche lobe.) Polars are distinguished from other CVs by the presence of a very strong magnetic field in the WD. Typical magnetic field strengths of polar systems are 10 million to 80 million gauss (1000–8000 teslas). The WD in the polar AN Ursae Majoris has the strongest known magnetic field among cataclysmic variables, with a field strength of 230 million gauss (23 kT). The presence of such a strong magnetic field gives rise to a number of fundamental differences between polars and non-magnetic CVs.

One of the most critical consequences of the WD's magnetism is that it synchronizes the rotational period of the WD with the orbital period of the binary; to first order, this means that the same side of the WD always faces the donor star. This synchronous rotation is considered a defining feature of polars. Additionally, the WD's magnetic field captures the accretion stream from the donor star before it can develop into an accretion disk. The capture of the accretion stream is known as threading, and it occurs when the magnetic pressure from the WD matches the stream's ram pressure. The captured material flows along the WD's magnetic field lines until it violently accretes onto the WD in a shock near one or more of the star's magnetic poles. This accretion region covers only a fraction of the WD's surface, but it can contribute half of the system's optical light. In addition to optical and near-infrared cyclotron radiation, the accretion region also produces X-rays due to the high temperature of gas within the shock, so polars are frequently brighter in X-rays than non-magnetic CVs.

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