Astronomy & Astrophysics

Free access article

A&A 418, L13-L16 (2004)
DOI: 10.1051/0004-6361:20040105

Letter

High-amplitude, long-term X-ray variability in the solar-type star HD 81809: The beginning of an X-ray activity cycle?

F. Favata¹, G. Micela², S. L. Baliunas³, J. H. M. M. Schmitt⁴, M. Güdel⁵, F. R. Harnden Jr.⁶, S. Sciortino² and R. A. Stern⁷

¹ Astrophysics Division - Research and Science Support Dept. of ESA, Postbus 299, 2200 AG Noordwijk, The Netherlands
² INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 15, Cambridge, MA 02138, USA
⁴ Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ Paul Scherrer Institut, Würenlingen and Villigen, 5235 Switzerland
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambdridge 02138 MA, USA
⁷ Lockheed Martin Advanced Technology Ctr., USA

(Received 4 February 2004 / Accepted 4 March 2004)

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.

Key words: stars: X-rays

Offprint request: F. Favata, Fabio.Favata@rssd.esa.int

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