Detecting prolonged activity minima in binary stars. The case of zeta2 Reticuli

Abstract

Context. It is well known that from 1645 to 1715 solar activity was notably low and the number of sunspots was extremely reduced. This epoch is known as the Maunder minimum (MM). The study of stars in prolonged activity minima such as the MM could help to shed light on this enigmatic epoch. However, to date, it is not easy to identify MM candidates among other stars. An original idea, which has hardly been explored, is to compare the activity levels of both components of binary systems. Aims: Our goal is to explore if the star ζ2 Ret, which belongs to a binary system, is in (or going to) a state similar to the MM. We have collected more than 430 spectra acquired between 2000 and 2019 with the HARPS, REOSC, UVES, and FEROS spectrographs. Methods: We performed a detailed long-term activity study of both components using the Mount Wilson index, which is obtained from the Ca II H&K lines. To search for signs of an activity cycle, we analysed the resulting time series with the Generalised Lomb-Scargle and CLEAN periodograms. Results: Our spectroscopic analysis shows a high activity level for ζ1 Ret and a significant decrease in the magnetic activity cycle amplitude of ζ2 Ret. Thus, the activity difference between both components has slightly increased (Δlog RHK' ~ 0.24 dex), when compared to the previously reported value. The long series analysed here allowed us to re-calculate and constrain the period of ζ2 Ret to ∼7.9 yr. We also detected a long-term activity cycle of ∼4.2 yr in ζ1 Ret, which has not been reported in the literature yet. Conclusions: By analogy with the scenario that proposes a weak solar cycle during the MM, we suggest that activity signatures by ζ2 Ret, that is to say a very low activity level when compared to its stellar companion, a notably decreasing amplitude (∼47%), and a cyclic behaviour, are possible evidence that this star could be in an MM state. To our knowledge, it is the first MM candidate star to have been detected through a highly discrepant activity behaviour in a binary system. Finally, we suggest that continuous observations of ζ2 could help to better understand unusual periods such as the MM.