The CARMENES search for exoplanets around M dwarfs. The He I triplet at 10830 $\mathrm{\AA}\,$ across the M dwarf sequence

TL;DR

This study investigates the properties of the He I IR triplet at 10830 Å across 319 M dwarf stars, revealing its behavior in absorption and emission related to stellar activity and temperature, with implications for understanding stellar chromospheres.

Contribution

It provides the first extensive analysis of the He I IR triplet in M dwarfs, showing its variation with spectral type and activity, and clarifying its formation mechanisms in these stars.

Findings

He I triplet appears in absorption in quiescent stars, decreasing with later spectral types.

Emission in the triplet is associated with stellar flares.

No correlation found between He I triplet strength and X-ray activity levels.

Abstract

The He I infrared (IR) triplet at 10830 AA 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 ($L_{\rm X}/L_{\rm bol}$ < -5.5) in our sample or to the complex behaviour with regard to increasing depth and filling in.