VLBI imaging of the RS CVn binary star system HR 5110

TL;DR

This paper presents VLBI images of the RS CVn binary HR 5110, revealing a core-halo emission structure, with the core showing flare activity and significant brightness increase over 12 hours.

Contribution

First VLBI imaging of HR 5110 showing detailed core-halo morphology and flare-related brightness changes in the binary system.

Findings

Core size is 0.39 mas, covering 66% of the star.

Halo size is 1.95 mas, about 1.8 times the star separation.

Brightness increased by ~200% during observations.

Abstract

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 chromospherically 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 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%.