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

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 (SMWO). 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 SMWO measurements and also include SMWO measurements from the historical Mount Wilson program. This results in an observational time baseline of over 50 yr for the SMWO data and of over 7 yr in X-rays. Moreover, we include Ca II infrared triplet (IRT) index measurements (SCa 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 SMWO 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 SMWO 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 SMWO measurements. The two shorter cycles are also detected in the SCa IRT data, although the activity cycles exhibit lower amplitudes in the SCa IRT than in the SMWO data. Finally, the rotation period of ϵ Eri can be found more frequently in the SMWO as well as in the SCa 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.