2020A&A...640A..52F

The HeI infrared (IR) triplet at 10830Å is known as an activity indicator in solar-type stars and has become a primary diagnostic in exoplanetary transmission spectroscopy. HeIIR lines are a tracer of the stellar extreme-ultraviolet irradiation from the transition region and corona. We study the variability of the HeI triplet lines in a spectral time series of 319 M dwarf stars that was obtained with the CARMENES high-resolution optical and near-infrared spectrograph at Calar Alto. We detect HeIIR line variability in 18% of our sample stars, all of which show Hα in emission. Therefore, we find detectable HeI variability in 78% of the sub-sample of stars with Hα emission. Detectable variability is strongly concentrated in the latest spectral sub-types, where the HeI lines during quiescence are typically weak. The fraction of stars with detectable HeI variation remains lower than 10% for stars earlier than M3.0 V, while it exceeds 30% for the later spectral sub-types. Flares are accompanied by particularly pronounced line variations, including strongly broadened lines with red and blue asymmetries. However, we also find evidence for enhanced HeI absorption, which is potentially associated with increased high-energy irradiation levels at flare onset. Generally, HeI and Hα line variations tend to be correlated, with Hα being the most sensitive indicator in terms of pseudo-equivalent width variation. This makes the HeI triplet a favourable target for planetary transmission spectroscopy.