A MAGNETOSPHERIC MODEL FOR RADIO-EMISSION FROM ACTIVE, LATE-TYPE BINARY STARS

A simple, two-dimensional model of the magnetosphere of late-type, active stars with strong, nonthermal radio emission is presented. The model assumes that the quiescent radio emission arises from a gyrosynchrotron process in a toroidal region of trapped plasma. This region ("dead zone") contains both thermal and relativistic plasma. The thermal component, which is also found in a "wind zone" exterior to the trapped plasma, is responsible for the observed X-ray emission, while the relativistic plasma is confined to the dead zone and has a power-law energy spectrum. The magnetic field is assumed to be strictly dipolar in the dead zone. We have solved the equation of radiative transfer along lines of sight lying in a plane containing the magnetic axis for pole-on and equator-on viewing angles. For these cases, symmetry allows an exact solution for the emergent I and V radiation. We calculated the propagation of the ordinary and extraordinary normal modes separately, taking into account possible mode coupling in regions of quasi-transverse magnetic field. We ignored refractive ray-bending effects. Using the observed angular size, spectrum and circular polarization characteristics of quiescent radio emission from RS CVn binary systems, we undertook a systematic search to determine the magnetic field strength, electron densities (thermal and relativistic), radial dependence, and shape of the electron energy spectrum which would reproduce the observed radio and X-ray properties. The model successfully reproduced many of the observations, including increasing circular polarization with inclination angle, polarization reversal between 1.4 and 5.0 GHz, and correct radio and X-ray luminosities.