${\zeta }^1+{\zeta }^2$ Reticuli binary system: A puzzling chromospheric activity pattern

Abstract

We perform, for the first time, a detailed long-term activity study of the binary system $\zeta$ Ret. We use all available HARPS spectra obtained between the years 2003 and 2016. We built a time series of the Mount Wilson $S$ index for both stars, then we analyse these series by using Lomb-Scargle periodograms. The components ${\zeta }^1$ Ret and ${\zeta }^2$ Ret that belong to this binary system are physically very similar to each other and also similar to our Sun, which makes it a remarkable system. We detect in the solar-analogue star ${\zeta }^2$ Ret a long-term activity cycle with a period of $\sim$10 yr, similar to the solar one ($\sim$11 yr). It is worthwhile to mention that this object satisfies previous criteria for a flat star and for a cycling star simultaneously. Another interesting feature of this binary system, is a high $\sim$0.220 dex difference between the averages log(${\mathrm{R}}_{\mathrm{HK}}^{\prime }$) activity levels of both stars. Our study clearly shows that ${\zeta }^1$ Ret is significantly more active than ${\zeta }^2$ Ret. In addition, ${\zeta }^1$ Ret shows an erratic variability in its stellar activity. In this work, we explore different scenarios trying to explain this rare behaviour in a pair of coeval stars, which could help to explain the difference in this and other binary systems. From these results, we also warn that for the development of activity-age calibrations (which commonly use binary systems and/or stellar clusters as calibrators) it should be taken into account the whole history of activity available of the stars involved.

9 pages and 8 figures

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