Polar (star)

Highly magnetic type of cataclysmic variable binary star system

In astronomy, a polar is a highly magnetic type of cataclysmic variable (CV) binary star system, originally known as an AM Herculis star after the prototype member AM Herculis. Like other 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, overflowing 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).

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; 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. The most common explanation for the WD's asynchronous rotation is that each of these systems had been synchronous until a nova eruption broke the synchronization by changing the WD's rotation period. The first known asynchronous polar, V1500 Cyg, underwent a nova in 1975, and its asynchronous rotation was discovered after the nova faded, providing the best observational evidence of this scenario. The corresponding variations in eclipse depth were shown to depend very strongly on the orientation of the WD's magnetic field with respect to the donor star. For comparison, in a synchronous polar, the WD does not rotate with respect to the donor star, and the stream always interacts with the same field lines, resulting in a stable accretion geometry. There is also evidence in each of the four asynchronous polars that the accretion stream is able to travel much deeper into the WD's magnetosphere than in synchronous systems, implying an unusually high rate of mass transfer from the donor star or a low magnetic field strength, but this has not been studied in detail.

Another class of cataclysmic variables with magnetic white dwarfs accreting material from a main sequence donor star are the intermediate polars. These have less strong magnetic fields and the rotation of the white dwarf is not synchronised with the orbital period. It has been proposed that intermediate polars may evolve into polars as the donor is depleted and the orbit shrinks.