2007A&A...466.1089B

We report on our analysis of the high resolution spectra (R≃86000) of a sample of 42 late-type active stars (with measured logR'_HK spanning from ≃-3 to ≃-5) acquired with the Italian 3.6m Telescopio Nazionale Galileo (TNG) using the SARG spectrometer in the 4960-10110Å 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 CaII infrared triplet (CaIIIRT) lines: the residual equivalent width (EQW) and the chromospheric indicator R_IRT. The aim of this work is determine and test the best way of deriving activity-level information and errors from the CaIIIRT lines, in preparation of the GAIA Cornerstone mission by ESA, by which the CaIIIRT spectral range will be spectroscopically observed for millions of stars. The R_IRT index is calculated for each observed star as the difference between the calculated NLTE photospheric central intensity and the observed one. The residual EQW, ΔW_IRT, is calculated as the area of the positive profile obtained as the difference between the calculated NLTE photospheric and the observed profiles. We correlate logR'_HK with R_IRT and the ΔW_IRT. This analysis indicates that CaIIIRT lines are good chromospheric diagnostics. We find that both ΔW_IRT and the R_IRT quantities can be used as chromospheric indicators, although the former exhibits a tighter correlation with the logR'_HK index. Furthermore, we find that the total chromospheric excess EQW in the CaIIIRT is almost linearly correlated with the excess in the CaIIH&K doublet, as estimated through the logR'_HK index.