The Ca II infrared triplet as a stellar activity diagnostic

Aims.We report on our analysis of the high resolution spectra () of a sample of 42 late-type active stars (with measured spanning from to ) acquired with the Italian 3.6 m Telescopio Nazionale Galileo (TNG) using the SARG spectrometer in the 4960-10 110 Å range. The high quality of the spectra and the good activity-level coverage allow us to measure two different chromospheric indicators that can be derived from the Ca ii infrared triplet (Ca ii IRT) lines: the residual equivalent width (EQW) and the chromospheric indicator . The aim of this work is determine and test the best way of deriving activity-level information and errors from the Ca ii IRT lines, in preparation of the GAIA Cornerstone mission by ESA, by which the Ca ii IRT spectral range will be spectroscopically observed for millions of stars.