The X-ray cycle in the solar-type star HD 81809. XMM-Newton observations and implications for the coronal structure

Context: The 11-yr cycle is the best known manifestation of the Sun's activity. While chromospheric cycles have been studied in a number of solar-like stars, very little is known about how these are reflected in the cyclical behavior of the coronal X-ray emission in stars other than the Sun.
Aims: Our long-term XMM-Newton program of long-term monitoring of a solar-like star with a well-studied chromospheric cycle, HD 81809, aims to study whether an X-ray cycle is present, along with studying its characteristics and its relation to the chromospheric cycle.
Methods: Regular observations of HD 81809 were performed with XMM-Newton, spaced by 6 months from 2001 to 2007. We studied the variations in the resulting coronal luminosity and temperature, and compared them with the chromospheric Ca ii variations. We also modeled the observations in terms of a mixture of active regions, using a methodology originally developed to study the solar corona.
Results: Our observations show a well-defined cycle with an amplitude exceeding 1 dex and an average luminosity approximately one order of magnitude higher than in the Sun. The behavior of the corona of HD 81809 can be modeled well in terms of varying coverage of solar-like active regions, with a larger coverage than for the Sun, showing it to be compatible with a simple extension of the solar case.