Stellar magnetosphere reconstruction from radio data - Multi-frequency VLA observations and 3D-simulations of CU Virginis

Aims. In order to fully understand the physical processes in the magnetospheres of the Magnetic Chemically Peculiar stars, we performed multi-frequency radio observations of CU Virginis. The radio emission of this kind of stars arises from the interaction between energetic electrons and magnetic field. Our analysis is used to test the physical scenario proposed for the radio emission from the MCP stars and to derive quantitative information about physical parameters not directly observable. Methods. The radio data were acquired with the VLA and cover the whole rotational period of CU Virginis. For each observed frequency the radio light curves of the total flux density and fraction of circular polarization were fitted using a three-dimensional MCP magnetospheric model simulating the stellar radio emission as a function of the magnetospheric physical parameters. Results. The observations show a clear correlation between the radio emission and the orientation of the magnetosphere of this oblique rotator. The radio emission is explained as the result of the acceleration of the wind particles in the current sheets just beyond the Alfvén radius, that eventually return toward the star following the magnetic field and emitting radiation by gyrosyncrotron mechanisms. The accelerated electrons we probed with our simulations have a hard energetic spectrum (N(E) ∝ E-2) and the acceleration process has an efficiency of about 10-3. The Alfvén radius we determined is in the range of 12-17 R* and, for a dipolar field of 3000 Gauss at the magnetic pole of the star, we determine a mass loss from the star of about 10-12 M⊙ yr-1. In the inner magnetosphere, inside the Alfvén radius, the confined stellar wind accumulates and reaches temperatures in the range of 105-106 K, and a detectable X-ray emission is expected.