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 luminosities of L _ { A } = 5.8 \pm 0.3 \mathcal { L } ^ { N } _ { \odot } and L _ { B } = 1.025 \pm 0.055 \mathcal { L } ^ { N } _ { \odot } , and radii R _ { A } = 2.42 \pm 0.08 \mathcal { R } ^ { N } _ { \odot } and R _ { B } = 1.04 \pm 0.04 \mathcal { R } ^ { N } _ { \odot } , confirming that the primary component is a subgiant .
We perform an independent estimate of the rotation period of the A component based on v \sin i and find that it agrees with the 40.2 d 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 ^ { \prime } _ { HK } ) \sim - 4.89 , while the secondary component has relatively flat activity at \log ( R ^ { \prime } _ { HK } ) \sim - 5.02 .