The He i infrared ( IR ) triplet at 10830 Å is an important activity indicator for the Sun and in solar-type stars , however , it has rarely been studied in relation to M dwarfs to date .
In this study , we use the time-averaged spectra of 319 single stars with spectral types ranging from M0.0 V to M9.0 V obtained with the CARMENES high resolution optical and near-infrared spectrograph at Calar Alto to study the properties of the He i IR triplet lines .
In quiescence , we find the triplet in absorption with a decrease of the measured pseudo equivalent width ( pEW ) towards later sub-types .
For stars later than M5.0 V , the He i triplet becomes undetectable in our study .
This dependence on effective temperature may be related to a change in chromospheric conditions along the M dwarf sequence .
When an emission in the triplet is observed , we attribute it to flaring .
The absence of emission during quiescence is consistent with line formation by photo-ionisation and recombination , while flare emission may be caused by collisions within dense material .
The He i triplet tends to increase in depth according to increasing activity levels , ultimately becoming filled in ; however , we do not find a correlation between the pEW ( He IR ) and X-ray properties .
This behaviour may be attributed to the absence of very inactive stars ( L _ { X } / L _ { bol } ¡ -5.5 ) in our sample or to the complex behaviour with regard to increasing depth and filling in .