AM Herculis Stars
The AM Herculis binaries have white dwarf primary stars with the strongest magnetic fields observed in the cataclysmic variables. They are unique in having no accretion discs and primary stars rotating in synchronism with the orbit. The accretion stream becomes magnetically channelled and reaches the primary by one or more localized columns. Unlike other cataclysmic variables, a significant fraction of AM Her binaries occupy the orbital period gap while undergoing mass transfer. These properties pose a range of MHD problems.
Inductive coupling to a tidally synchronized secondary leads to a synchronization torque, but the maintenance of a synchronous state requires non-dissipative torques to act. The 3D nature of the channelled accretion stream results in an accretion torque which has components parallel and normal to the orbital angular momentum vector, leading to restrictions on the nature of the required balancing torque. Certain conditions are necessary for the attainment of synchronism. Asynchronous rotation of the primary can significantly affect the mass transfer rate, due to magnetic spin-orbit coupling. The essential properties of the AM Her binaries are described here, together with a list of the confirmed systems.
- Bailey, J.A., Wickramasinghe, D.T., Hough, J.H., Cropper, M.S., 1988, MNRAS, 234, 19P.Google Scholar
- Beuermann, K., Thomas, H.C., Schwope, A.D., 1989, IAUC., 4775.Google Scholar
- Katz, J.H., 1991, ComAp, 15, No. 4, 177.Google Scholar
- Littlefield, C., Garnavich, P., Magno, K., Murison, M., deal, S., McClelland, C., Rose, B., 2015a, IBVS, 6129, 1.Google Scholar
- Schwarz, R., Greiner, J., 1999, PASC, 157, 139.Google Scholar
- Schwope, A.D., Mackebrandt, Thinius, B.D., Littlefield, C., Garnavich, P., Oksanen, A., Granzer, T., 2015, AN, 336, 115.Google Scholar
- Tapia, S., 1979, IAUC., 3327, 2.Google Scholar
- Tovmassian, G.A., Szkody, P., Greiner, j., Zharikov, S.V., Zickgraf, F.J., Serrano, A., Krautter, J., Thiering, I., Neustroev, V., 2001, A&A, 379, 199.Google Scholar