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Context. Radial velocities (RVs) of stars contain both the Doppler reflexmotion of potential planetary companions and the drowning and sometimesimitating effect of stellar activity. To separate the two, previous effortshave sought for proxys which only trace the activity signals, yet thesub-meter-per-second floor required for the detection of Earth-like planetsremains difficult to break. Aims. In this work, we analyze a sample of 12 G- toearly M-type stars in order to investigate the feasibility of detecting adifferential effect of stellar activity with photospheric depth, as traced bythe spectral line-forming temperature, for observations with different samplingand noise levels. Methods. We computed the average line formation temperaturefor each point in the observed wavelength grids using the spectral synthesiscode PySME. The final line selection was curated to exclude blended and poorlysynthesized lines. We thereafter computed the convective blueshift (CB) of theline cores of our master spectra (composed of the stacked individual spectrafor each star). Finally, we extract RV time series for certain intervals offormation temperature using a template-matching approach. Results. We find theCB to follow a linear relation with the formation temperature of the linecores, and the CB slope to be steeper with increasing effective temperature.For the RV time series derived for different intervals of formationtemperature, we find the RVs of line parts formed at higher temperatures, closeto the spectral continuum, to be generally correlated with the S index, and RVsof line parts formed at cooler temperatures, close to the spectral line cores,to be generally anti-correlated, especially for stars with low noise levels andsignificant variations over their magnetic cycles.

Measuring precise radial velocities on individual spectral lines. IV. Stellar activity correlation with line formation temperature