Deconvolving the HD 81809 binary: Rotational and activity evidence for a subgiant with a sun-like cycle

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-circle dot(N), and L-B = 1.025 +/- 0.055 L-circle dot(N), and radii R-A 2.42 +/- 0.08 R-circle dot(N) and R-B 1.04 +/- 0.04 R-circle dot(N), 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 days period previously measured from the Ca HK time series.