Multiple and changing cycles of active stars

II. Results

¹ Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, Hungary e-mail: olah@konkoly.hu
² Astrophysical Institute Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁴ Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
⁵ Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland
⁶ ESO, Karl-Schwarzschild Strasse 2, 85748 Garching bei Munchen, Germany
⁷ Harvard - Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA

Received: 6 November 2008
Accepted: 27 March 2009

Abstract

Aims. We study the time variations in the cycles of 20 active stars based on decade-long photometric or spectroscopic observations.

Methods. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data.

Results. Fifteen stars definitely show multiple cycles, but the records of the rest are too short to verify a timescale for a second cycle. The cycles typically show systematic changes. For three stars, we found two cycles in each of them that are not harmonics and vary in parallel, indicating a common physical mechanism arising from a dynamo construct. The positive relation between the rotational and cycle periods is confirmed for the inhomogeneous set of active stars.

Conclusions. Stellar activity cycles are generally multiple and variable.