Detecting prolonged activity minima in binary stars. The case of Zet02 Ret

MATÍAS FLORES; MARCELO JAQUE ARANCIBIA; ROMINA IBAÑEZ BUSTOS; ANDREA BUCCINO; J. YANA GALARZA; NATALIA NUÑEZ; PAULA MIQUELARENA; JOSÉ ALACORIA; CARLOS SAFFE; PABLO MAUAS

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2021

0004-6361

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  Zet02 Ret, which belongs to a binary system, is in (or going to) a state similar to the MM. We havecollected 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 fromthe Ca II H&K lines. To search for signs of an activity cycle, we analysed the resulting time series with the Generalised Lomb-Scargleand CLEAN periodograms.Results. Our spectroscopic analysis shows a high activity level for  Zet01 Ret and a significant decrease in the magnetic activity cycleamplitude of  Zet02 Ret. Thus, the activity difference between both components has slightly increased (∆log RHK ∼0.24 dex), whencompared to the previously reported value. The long series analysed here allowed us to re-calculate and constrain the period of  Zet02 Retto ∼ 7.9 yr. We also detected a long-term activity cycle of ∼4.2 yr in  Zet01 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 Zet02 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  Zet02 could help to better understand unusual periods such as the MM.