2002ApJ...572..487R

We present the first VLBI images of the RS CVn binary star HR 1099 (=V711 Tauri, HD 22468) obtained from observations at 8.4 GHz in 1996 May and September made in support of the NASA/Stanford Gravity Probe B project. The first set of observations was made during a decay stage of a flare event. The second set of observations was made during a quiescent period. The detected emission region for the active epoch had an estimated major-axis length (FWHM) of 2.7±0.2 mas. This region consisted of a halo and two superimposed compact condensations that were oriented approximately east-west. The centers of the compact condensations were separated by 1.7±0.1 mas. Compared to the ∼1.3 mas separation on the sky at this epoch of the centers of the stellar components of the binary, the observed separation of the condensations differs by only 0.4 mas, which is less than the ∼0.6 mas angular radius of the larger component. During the observations, the compact western condensation rotated north-northwestward by 24°±4°, or by 0.7±0.1 mas, relative to the eastern condensation. We speculate that (1) either both condensations originate from the corona of the larger stellar component or one condensation is close to the surface of each of the two stellar components of the binary and (2) the relative rotation of the two condensations is a consequence of the rotation of the binary system. We speculate further that the halo is a consequence of flare-energized electrons confined by the magnetosphere of the larger stellar component or by the combined magnetospheres of the two stellar components. Our quiescent epoch, in contrast, was characterized by a single emission region with a major axis estimated to be 1.7±0.1 mas (FWHM).