A multi-wavelength view of the multiple activity cycles of ϵ Eridani

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: bfuhrmeister@hs.uni-hamburg.de
² Institut für Astronomie und Astrophysik Tübingen, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

Received: 13 October 2022
Accepted: 8 March 2023

Abstract

ϵ Eridani is a highly active young K2 star with an activity cycle of about 3 yr established using Ca II H& K line index measurements (S_MWO). This relatively short cycle has been demonstrated to be consistent with X-ray and magnetic flux measurements. Recent work suggested a change in the cyclic behaviour. Here, we report new X-ray flux and S_MWO measurements and also include S_MWO measurements from the historical Mount Wilson program. This results in an observational time baseline of over 50 yr for the S_MWO data and of over 7 yr in X-rays. Moreover, we include Ca II infrared triplet (IRT) index measurements (S_{Ca IRT}) from 2013 to 2022 in our study. With the extended X-ray data set, we can now detect the short cycle for the first time using a periodogram analysis. Near-simultaneous S_MWO data and X-ray fluxes, which are offset by 20 days at most, are moderately strongly correlated when only the lowest activity state (concerning short-term variability) is considered in both diagnostics. In the S_MWO data, we find strong evidence for a much longer cycle of about 34 yr and an 11-yr cycle instead of the formerly proposed 12-yr cycle in addition to the known 3-yr cycle. The superposition of the three periods naturally explains the recent drop in S_MWO measurements. The two shorter cycles are also detected in the S_{Ca IRT} data, although the activity cycles exhibit lower amplitudes in the S_{Ca IRT} than in the S_MWO data. Finally, the rotation period of ϵ Eri can be found more frequently in the S_MWO as well as in the S_{Ca IRT} data for times near the minimum of the long cycle. This may be explained by a scenario in which the filling factor for magnetically active regions near cycle maximum is too high to allow for notable short-term variations.