Multiple and changing cycles of active stars

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.

Stellar magnetic cycles

“…Stellar cycles can be associated with quasi-sinusoidal or highly anharmonic variations of the proxies, they show single or multiple periodicities, and their periods may change versus time (cf. Oláh et al 2009). The relationship of the cycle period to the stellar parameters is by no means a simple one.…”

“…Moreover, changing of the cycle period as well as multiperiodic variations are often observed (cf. Oláh et al 2009), which may be related to the complex feedback mechanisms operating in a highly non-linear stellar dynamo (cf. Spiegel 2009).…”

“…Since stellar cycles are manifestations of stellar dynamos whose regimes mainly depend on the dynamo number D, most of the studies have focussed on the relationship between the period of the cycle and the rotation period, or the Rossby number, assumed to be a better proxy for D. A plot of the cycle period P cyc vs. the rotation period P rot shows a large scatter, particularly for P rot < 10 − 20 days and a generally weak correlation between P cyc and P rot (cf., e.g., Saar & Brandeburg 1999;Oláh et al 2009). When the cycle period is measured in units of the rotation period of the star and plotted vs. the Rossby number, the correlation improves if the stars are subdivided into three branches (cf.…”

The Solar-Stellar Connection

“…However, significant uncertainties in our current understanding of the solar dynamo mechanism motivates us to confront our theoretical ideas of magnetic field generation with the wider space parameters available from stellar activity observations (Noyes et al 1984a;Baliunas et al 1995;Giampapa 2005;Böhm-Vitense 2007;Oláh et al 2009;Donati and Landstreet 2009;Reiners 2012, and references therein). While, in principle, different stars with different properties may host distinct classes of MHD dynamos, a good starting assumption is that a reasonable model of solar and stellar magnetism should be able to reproduce the range of stellar activity taking into account appropriate changes in the internal driving mechanisms.…”

The chromospherically active binary star EI Eridani: II. Long‐term Doppler imaging

“…They did not discuss a possible change in the level of spottedness itself, though. Oláh et al (2000) favoured a length of 16.2 years on a time base of 18.5 years and quote a significant remaining period of 2.4 years. Adding four more years of data, the preferred cycle length was back to 12.2 years (see Oláh & Strassmeier 2002).…”