2018ApJ...866...80E

HD 81809 has one of the highest quality activity cycles from the sample of stars synoptically observed in the Mount Wilson Observatory HK Project. However, this object is in fact a binary system, raising the question as to which of the components is responsible for the observed cyclic activity and what are the properties of that active component. The Hipparcos spacecraft obtained resolved two-color photometry for this system that indicates that both components are near the solar temperature. Combined with the precise Gaia parallax and empirical bolometric corrections we derive component the luminosities of L_A=5.8±0.3 L_☉^N and L_B=1.025±0.055 L_☉^N, and radii R_A=2.42±0.08 R_☉^N and R_B=1.04±0.04 R_☉^N, confirming that the primary component is a subgiant. We perform an independent estimate of the rotation period of the A component based on vsini and find that it agrees with the 40.2 days period previously measured from the Ca HK time series. We explore plausible scenarios for the deconvolved S-index and find that a cycling A component would have an activity level within the bounds of ensemble activity-rotation trends, while a cycling B component likely does not. Based on the available rotation and activity evidence, we find the most likely characterization of the system is a subgiant primary component responsible for the smooth cyclic behavior in Ca HK with log(R_HK^′^)∼-4.89, while the secondary component has relatively flat activity at log(R_HK^′^)∼-5.02.