Stellar activity with LAMOST. III. Temporal variability pattern in Pleiades, Praesepe, and Hyades

TL;DR

This study analyzes the temporal variability of stellar activity in young stars within three open clusters using LAMOST spectra, revealing diverse activity patterns, correlations, and implications for stellar magnetic behavior.

Contribution

It provides the first systematic analysis of multi-epoch spectral variability in young cluster stars, linking activity changes to rotation and age.

Findings

Active stars show larger variability in activity indicators.

Anti-correlation between TiO2 and Hα variations suggests linked activity regions.

Some M dwarfs exhibit co-located plages and starspots.

Abstract

We present the results from a systematic study of temporal variation of stellar activity in young late-type stars. We used multi-epoch LAMOST low-resolution spectra of over 300 member candidates in three young open clusters: Pleiades, Praesepe, and Hyades. The spectral measurements of TiO band strength near 7050 {\AA} (TiO2) and equivalent width of H{\alpha} line (EW H{\alpha} ) are used as the tracers of cool-spot coverage and chromospheric emission strength, respectively. The analysis of time-variation patterns of these two tracers suggested there exist detectable variabilities in TiO2 and EW H{\alpha} , and their time-scales are in the wide range from days to years. Results showed that more active stars, younger and fast rotators, tend to have larger activity variations. There is a tendency of anti-correlation between temporal variations in TiO2 and EW H{\alpha} . Also, appreciable anti-correlation in the rotational phase between H{\alpha} emission and K2 brightness is detected in some M dwarfs, indicating spatial co-location of the plages with cool starspots, however, cool stars do not always show such co-location features. Furthermore, spot coverage and H{\alpha} emission were evident at all rotational phases of several M dwarfs, indicating a basal level of activity, perhaps due to many small and randomly located active regions in the atmosphere.