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

TL;DR

This study used radio interferometry to precisely measure the distance to the binary system AM Herculis and analyzed its radio emission properties, revealing modulation linked to orbital motion and magnetic activity.

Contribution

First precise parallax measurement of AM Herculis using VLBI, and analysis of its radio emission modulation related to orbital and magnetic phenomena.

Findings

Parallax of 11.29±0.08 mas, distance 88.6±0.6 pc.

Radio flux modulated with orbital motion.

Radio emission likely originates near the red dwarf's surface.

Abstract

We conducted radio-interferometric observations of the well known binary cataclysmic system AM Herculis. This particular system is formed by the magnetic white dwarf (primary) and the red dwarf (secondary), and is the prototype of so-called polars. Our observations were conducted with the European VLBI Network (EVN) in the e-EVN mode at 5 GHz. We obtained six astrometric measurements spanning one year, which make it possible to update the annual parallax for this system with the best precision to date ($\pi=11.29\pm0.08$ mas), equivalent to the distance of $88.6\pm0.6$ pc. The system was observed mostly in the quiescent phase (visual magnitude $m_v\sim15.3$), when the radio emission was at the level of about $300~\mu$Jy. 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 the emission mechanism similar to the scenario proposed for V471 Tauri 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 together with AR Ursae Majoris (the second known persistent radio polar) from other polars, as the systems with a magnetized secondary star.