Discovery of an activity cycle in the solar-analog HD 45184. Exploring Balmer and metallic lines as activity proxy candidates

TL;DR

This study detects a 5.14-year activity cycle in the solar-analog star HD 45184 using optical lines beyond Ca II H&K, revealing new activity proxies like Fe II lines and their correlation with stellar cycles.

Contribution

It is the first to identify a long-term activity cycle in HD 45184 using Balmer and metallic lines, expanding the methods for stellar activity detection beyond traditional proxies.

Findings

HD 45184 has a 5.14-year activity cycle.

Balmer lines show variability correlated with the cycle.

Fe II lines are modulated by the star's activity cycle.

Abstract

(Abridged) Most stellar activity cycles similar to that found in the Sun have been detected by using the chromospheric Ca II H&K lines as stellar activity proxies. However, it is unclear if such activity cycles could be identified using other optical lines. Aims. To detect activity cycles in solar-analog stars and determine if these can be identified through other optical lines, such as Fe II and Balmer lines. We study the solar-analog star HD 45184. Methods. We analyse the activity signatures of HD 45184 by using 291 HARPS spectra obtained between 2003 and 2014. In order to search for line-core fluxes variations, we focus on Ca II H&K and Balmer H$\alpha$, H$\beta$ lines, which are usually used as optical chromospheric activity indicators. We calculate the HARPS-S index from Ca II H&K lines and convert it to the Mount-Wilson scale. In addition, we also consider as activity indicators the equivalent widths of Balmer lines. Moreover, we analyse the possible variability of Fe II and other metallic lines in the optical spectra. Results. We report for the first time a long-term 5.14-yr activity cycle in the solar-analog star HD 45184. This makes HD 45184 one of most similar stars to the Sun with known activity cycle. Such variation is also evident in the first lines of the Balmer series, which not always show a correlation with activity in solar-type stars. Notably, unlike the solar case, we also found that the equivalent widths of the high photospheric Fe II lines (4924 \AA, 5018 {\AA} and 5169 \AA) are modulated ($\pm$ 2 m\AA) by the chromospheric cycle of the star. From short-term modulation of the S index we calculate a rotational period of 19.98 days, which agrees with its mean chromospheric activity level.