Rotational and Activity Evidence for a Subgiant with a Sun-Like Cycle

Wednesday, October 10, 2018

HD 81809 has one of the highest-quality activity cycles from the sample of stars synoptically observed in the Mount Wilson Observatory HK Project. Its Sun-like 8.2 year cycle and slow 40.2 day rotation period can be clearly seen in over 50 years of observations of the calcium HK line emission, a proxy for magnetic flux. 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. Previous work had concluded that the activity cycle is due to a K-type component, significantly cooler than the Sun.

HD 81809 binary system in calcium HK emission image
The Sun-like activity cycle of HD 81809 binary system in calcium HK emission from Mount Wilson Observatory (red) and the Lowell Observatory Solar-Stellar Spectrograph (blue), as well as XMM-Newton X-ray observations (white). A periodogram analysis of the data (bottom) shows a clear peak at 8.2 years. New evidence reveals that this activity cycle is likely coming from the 5.8 solar-luminosity subgiant component of the system.

The Hipparcos spacecraft obtained resolved two-color photometry for this system that indicates that both components are near the solar temperature. These observation further allow us to derive the radius and luminosity of the individual components. We find that the primary component is a subgiant with 5.8 times the solar luminosity, and the secondary is Sun-like with approximately the solar luminosity. We perform an independent estimate of the rotation period of the primary component based on the doppler broadening of the convolved spectra and find that it agrees with the 40.2 day period previously measured from the calcium HK time series. We also explore plausible scenarios for the deconvolved calcium HK activity and find that a cycling primary component would have an activity level within the bounds of ensemble activity-rotation trends, while a cycling secondary component likely does not.

Based on the available rotation and activity evidence, we find the most likely characterization of the system is a luminous subgiant primary component responsible for the smooth 8.2 year cyclic behavior in calcium HK, while the Sun-like secondary component has relatively flat activity.

Publication Name: The Astrophysical Journal
First HAO Author's Name: Ricky Egeland