Very long baseline interferometry of chromospherically active binary stars

Stars with lower atmospheric temperatures, our own Sun among them, sometimes exhibit surprising signs of an atmosphere out of thermal equilibrium. These signs include H and K line emission, large dark spotted regions, enhanced non-thermal radio emission, and X-ray emission. All of these signs are indicative of chromospheric activity, and among the most active stars are the short-period chromosopherically active binaries. The activity in these binary stars is thought to come from the strong stellar magnetic dynamo driven by their rapid rotation. For these binary systems, is the non-thermal radio emission associated with a preferred region within the binary system or is the emission distributed more evenly across one or both stars? Very long baseline interferometry (VLBI) offers the most direct probe of the non-thermal inner binary environment with an angular resolution comparable to or smaller than the angular size of the component stars and binary separation. Here I present two VLBI studies of magnetically active binary stars, HR 5110 and HR 1099, where we attempt to associate the radio emission region with a region that is fixed in the frame of reference which is corotating with the binary system. For the HR 5110 observing campaign the emission is best associated with a region corotating with a semi-major axis that closely matches one of the companion stars. This confirms the region of activity is one of the stars in the system and opposed to an interaction region between the component stars. For the HR 1099 observing campaign there are hints of motion within each observing epoch. I discuss the practical limits of phase-referenced interferometry with respect to the achievable astrometric accuracy. Finally, I outline a blueprint for a future VLBI campaign to observe the kinematics of the radio emission region in the context of the binary system.