The X-ray cycle in the solar-type star HD 81809

TL;DR

This study presents evidence of a clear X-ray cycle in the solar-like star HD 81809, showing larger amplitude and higher luminosity than the Sun, and models its coronal activity as varying active region coverage.

Contribution

It provides the first long-term X-ray monitoring of HD 81809, demonstrating a solar-like cycle and modeling its coronal activity with active region coverage analysis.

Findings

HD 81809 exhibits a well-defined X-ray cycle with >1 dex amplitude.

The star's average coronal luminosity is about ten times solar.

Coronal variations are consistent with changing active region coverage.

Abstract

(abridged) 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. 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 CaII variations. We also modeled the observations in terms of a mixture of active regions, using a methodology originally developed to study the solar corona. 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.