Magnetic cycles at different ages of stars

¹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 1121 Budapest, Konkoly Thege M. u. 15-17, Hungary
e-mail: olah@konkoly.hu
² Eötvös University, Department of Astronomy, Pf. 32, 1518 Budapest, Hungary
³ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁴ c/o Dr. W. Soon, Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁵ University of West Hungary, Savaria Campus, Institute of Mathematics and Physics, Sopron 9400, Hungary

Received: 10 March 2016
Accepted: 20 April 2016

Abstract

Aims. We study the different patterns of interannual magnetic variability in stars on or near the lower main sequence, approximately solar-type (G-K dwarf) stars in time series of 36 yr from the Mount Wilson Observatory Ca ii H&K survey. Our main aim is to search for correlations between cycles, activity measures, and ages.

Methods. Time-frequency analysis has been used to discern and reveal patterns and morphology of stellar activity cycles, including multiple and changing cycles, in the datasets. Both the results from short-term Fourier transform and its refinement using the Choi-Williams distribution, with better frequency resolution, are presented in this study. Rotational periods of the stars were derived using multifrequency Fourier analysis.

Results. We found at least one activity cycle on 28 of the 29 stars we studied. Twelve stars, with longer rotational periods (39.7 ± 6.0 days), have simple smooth cycles, and the remaining stars, with much faster rotation (18.1 ± 12.2 days) on average, show complex and sometimes vigorously changing multiple cycles. The cycles are longer and quite uniform in the first group (9.7 ± 1.9 yr), while they are generally shorter and vary more strongly in the second group (7.6 ± 4.9). The clear age division between stars with smooth and complex cycles follows the known separation between the older and younger stars at around 2 to 3 Gyr of age.