High-amplitude, long-term X-ray variability in the solar-type star HD 81809: The beginning of an X-ray activity cycle?
Open Access
- 2 April 2004
- journal article
- letter
- Published by EDP Sciences in Astronomy & Astrophysics
- Vol. 418 (1), L13-L16
- https://doi.org/10.1051/0004-6361:20040105
Abstract
We present the initial results from our XMM-Newton program aimed at searching for X-ray activity cycles in solar-type stars. HD 81809 is a G2-type star (somewhat more evolved than the Sun, and with a less massive companion) with a pronounced 8.2 yr chromospheric cycle, as evident from from the Mt. Wilson program data. We present here the results from the initial 2.5 years of XMM-Newton observations, showing that large amplitude (a factor of $simeq$10) modulation is present in the X-ray luminosity, with a clearly defined maximum in mid 2002 and a steady decrease since then. The maximum of the chromospheric cycle took place in 2001; if the observed X-ray variability is the initial part of an X-ray cycle, this could imply a phase shift between chromospheric and coronal activity, although the current descent into chromospheric cycle minimum is well reflected into the star's X-ray luminosity. The observations presented here provide clear evidence for the presence of large amplitude X-ray variability coherent with the activity cycle in the chromosphere in a star other than the Sun.Keywords
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