An Etymological Dictionary of Astronomy and Astrophysics

A → red dwarf variable star located in the constellation → Hercules. AM Herculis usually remains in its “high” state (V ~ 13 mag), but from time to time it drops into a “low” state (V> 15 mag) that may last weeks or months. Originally classified as an irregular variable and associated with the X-ray source 3U 1809+50, AM Her was discovered in 1976 to be a short-period → binary system (3.1 hour orbital period) related to the → cataclysmic variables. It is in fact an → interacting binary in which a red dwarf (M4 V) loses mass to a → white dwarf primary star. The white dwarf has a very strong magnetic field (B ~ 2 × 10⁸ G) and rotates synchronously with the orbit. The magnetic field constrains the → mass loss from the secondary star to flow along a stream directly onto the white dwarf rather than into an → accretion disk as in the nonmagnetic cataclysmic variables. The optical spectrum shows strong emission lines of H, He I, and He II, along with weaker lines of other ions such as N III, C III, C II, and Ca II, all showing large velocity variations. → TiO bands from the M4 V secondary star are detected when the system is in its “low” state. AM Her is the class prototype of the → polars (see, e.g., Hutchings et al. 2002, AJ 123, 2841).

See also: AM, letters of alphabet used in variable star designations; → Hercules.