VLBI imaging of the RS CVn binary star system HR 5110

We present VLBI images of the RS CVn binary star HR 5110 (=BH CVn; HD 118216), obtained from observations made at 8.4 GHz on 1994 May 29/30 in support of the NASA/Stanford Gravity Probe B project. Our images show an emission region with a core-halo morphology. The core was 0.39 ± 0.09 mas (FWHM) in size, or 66%±20% of the 0.6 ±0.1 mas diameter of the chromospher-ically active K subgiant star in the binary system. The halo was 1.95 ± 0.22 mas (FWHM) in size, or 1.8 ± 0.2 times the 1.1 ± 0.1 mas separation of the centers of the K and F stars. (The uncertainties given for the diameter of the K star and its separation from the F star each reflect the level of agreement of the two most recent published determinations.) The core increased significantly in brightness over the course of the observations and seems to have been the site of flare activity that generated an increase in the total flux density of ∼200% in 12 hours. The fractional circular polarization simultaneously decreased from ∼10% to 2.5%. – 2 –