Chromospheric modeling for CARMENES targets with PHOENIX

Stellar activity is ubiquitous in M dwarfs. It is a notorious nuisance for high-precision radial
velocity planet searches and produces hazardous environments for the atmospheres of planets
orbiting these stars. The characterization of the temperature structure of the chromosphere is
imperative to understand the upper atmosphere of late-type stars and its impact on their observed
spectra. Using the stellar atmosphere code PHOENIX, we constructed a set of 1D chromospheric
models with empirical temperature structure to simultaneously fit the lines of Na I D₂, H\(\alpha\), and the
bluest Ca II infrared triplet line in a sample of 50 M2-3 dwarfs observed by CARMENES (Calar
Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle
Spectrographs). For the inactive stars, we find good agreement with individual models with a
temperature structure corresponding to modified VAL C models. Reproducing the chromospheric
lines of the active stars requires linear combinations of an inactive and an active model component,
from which we also obtain surface filling factors for the respective components. For stars observed
at different activity levels, our modeling yields a significant correlation between the surface filling
factor of the active model component and the activity level. Although the models are not optimized
for the He I infrared line at 10 830 Å, the best-fit models also give an adequate prediction of the
respective line absorption for most stars of the stellar sample. Furthermore, we performed a
detailed analysis of the He I infrared line behavior in prescribed model configurations with different
levels of activity. The behavior of the He I infrared line strengths as a function of the respective
EUV radiation shows the need of the mechanism of photoionization and recombination to form the
line for inactive models, while collisions start to contribute for more active models.