Another look at AM Herculis – radio-astrometric campaign with the e-EVN at 6 cm

Abstract We conducted radio-interferometric observations of the well-known binary cataclysmic system AM Herculis. This particular system is formed from a magnetic white dwarf (primary) and a red dwarf (secondary), and it is the prototype of so-called polars. Our observations were conducted with the European VLBI Network (EVN) in e-EVN mode at 5 GHz. We obtained six astrometric measurements spanning 1 yr, which make it possible to update the annual parallax for this system with the best precision to date (π = 11.29 ± 0.08 mas), which is equivalent to a distance of 88.6 ± 0.6 pc. The system was observed mostly in the quiescent phase (visual magnitude mv ∼ 15.3), when the radio emission was at the level of about 300 μJy. Our analysis suggests that the radio flux of AM Herculis is modulated with the orbital motion. Such specific properties of the radiation can be explained using an emission mechanism like the scenario proposed for V471 Tau and, in general, for RS CVn-type stars. In this scenario, the radio emission arises near the surface of the red dwarf, where the global magnetic field strength may reach a few kG. We argue that the quiescent radio emission distinguishes AM Herculis and AR Ursae Majoris (a second known persistent radio polar) from other polars, which are systems with a magnetized secondary star.