Long-term stellar activity variations of stars from the HARPS M-dwarf sample: Comparison between activity indices

TL;DR

This study analyzes long-term stellar activity variations in 23 M-dwarf stars using four chromospheric activity indicators, revealing strong correlations between some indices and activity levels, which aids understanding stellar magnetic activity.

Contribution

The paper compares multiple activity indices over long-term observations of M-dwarfs, highlighting their correlations and dependencies on activity levels, which was not previously detailed.

Findings

Strong correlation between $S_{CaII}$ and ext{Na}{i} indices.

Correlation between $S_{CaII}$ and H$eta$ depends on activity level.

No significant correlation between $S_{CaII}$ and ext{He}{i} indices.

Abstract

We used four known chromospheric activity indicators to measure long-term activity variations in a sample of 23 M-dwarf stars from the HARPS planet search program. We compared the indices using weighted Pearson correlation coefficients and found that in general (i) the correlation between $S_{CaII}$ and \ion{Na}{i} is very strong and does not depend on the activity level of the stars, (ii) the correlation between our $S_{CaII}$ and H$\alpha$ seems to depend on the activity level of the stars, and (iii) there is no strong correlation between $S_{CaII}$ and \ion{He}{i} for these type of stars.