The CARMENES search for exoplanets around M dwarfs. Variability of the He I line at 10830 \AA

TL;DR

This study investigates the variability of the He I IR triplet at 10830 Å in 319 M dwarf stars, revealing that variability is common in active stars, especially in later spectral types, and is often associated with stellar flares and Hα emission.

Contribution

It provides the first large-scale analysis of He I IR triplet variability in M dwarfs, linking it to stellar activity and flare phenomena, and assesses its potential for exoplanet transmission spectroscopy.

Findings

He I IR variability detected in 18% of stars, mainly in active, late-type M dwarfs.

Flares cause significant line broadening and asymmetries in He I IR lines.

He I IR variability correlates with Hα emission, especially in later spectral sub-types.

Abstract

The He I infrared (IR) triplet at 10830 \AA is known as an activity indicator in solar-type stars and has become a primary diagnostic in exoplanetary transmission spectroscopy. He I lines are a tracer of the stellar extreme-ultraviolet irradiation from the transition region and corona. We study the variability of the He I IR 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 He I IR line variability in 18% of our sample stars, all of which show H$\alpha$ in emission. Therefore, we find detectable He I IR variability in 78% of the sub-sample of stars with H$\alpha$ emission. Detectable variability is strongly concentrated in the latest spectral sub-types, where the He I IR lines during quiescence are typically weak. The fraction of stars with detectable He I IR 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 He I IR absorption, which is potentially associated with increased high-energy irradiation levels at flare onset. Generally, He I IR and H$\alpha$ line variations tend to be correlated, with H$\alpha$ being the most sensitive indicator in terms of pseudo-equivalent width variation. This makes the He I IR triplet a favourable target for planetary transmission spectroscopy.