2017A&A...607A..87M

Aims. Previous studies have shown that, for late-type stars, activity indicators derived from the CaII infrared-triplet (IRT) lines are correlated with the indicators derived from the CaII H&K lines. Therefore, the CaII IRT lines are in principle usable for activity studies, but they may be less sensitive when measuring the rotation period. Our goal is to determine whether the CaII IRT lines are sufficiently sensitive to measure rotation periods and how any CaII IRT derived rotation periods compare with periods derived from the "classical" Mount Wilson S-index.
Methods. To analyse the CaII IRT lines' sensitivity and to measure rotation periods, we define an activity index for each of the CaII IRT lines similar to the Mount Wilson S-index and perform a period analysis for the lines separately and jointly.
Results. For eleven late-type stars we can measure the rotation periods using the CaII IRT indices similar to those found in the Mount Wilson S-index time series and find that a period derived from all four indices gives the most probable rotation period; we find good agreement for stars with already existing literature values. In a few cases the computed periodograms show a complicated structure with multiple peaks, meaning that formally different periods are derived in different indices. We show that in one case, this is due to data sampling effects and argue that denser cadence sampling is necessary to provide credible evidence for differential rotation. However, our TIGRE data for HD 101501 shows good evidence for the presence of differential rotation.