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

TL;DR

This study presents the first detailed long-term analysis of the binary system $$ Ret, revealing a solar-like activity cycle in one star and significant activity differences between the two coeval stars, with implications for stellar activity calibration.

Contribution

This work provides the first long-term activity analysis of the $$ Ret binary system, highlighting complex activity patterns and differences that impact stellar age calibration methods.

Findings

$$ Ret $ Ret show a 10-year activity cycle.

$$ Ret is more active and erratically variable than $$ Ret.

Significant activity level difference (~0.220 dex) between the two stars.

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}$) 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.