The CARMENES search for exoplanets around M dwarfs: Line-by-line sensitivity to activity in M dwarfs

Stellar activity poses one of the main obstacles for the detection and characterisation of exoplanets around cool stars, as it deforms the absorption line profiles of stellar spectra creating signals that can hide or mimic the presence of companions. A wide variety of techniques are being developed with the aim of disentangling signals of stellar and planetary origins. These range from the decorrelation or modelling of the activity signals in the radial velocity (RV) time series, to the RV extraction using carefully selected wavelength regions of the stellar spectrum which are less affected by activity. In this work, we analyse the effects of activity on individual absorption features present on the optical spectra of several active M dwarf stars observed with the CARMENES high-resolution spectrograph. By studying the correlations between the individual line RVs and activity indicators, we are able to classify the observed lines according to their sensitivity to activity. This allows us to select differently affected lines and use them to compute RVs for which we mitigate or enhance the activity signal to varying degrees. We also observe that the same lines on similar stars show different sensitivities to activity, suggesting the generalisation of a line list a challenging endeavour.