Chromospheres of late-type active and quiescent dwarfs. II. An activity index derived from profiles of the Ca II λ 8498 A and λ 8542 A triplet lines

High resolution high signal to noise spectral profiles of the λ 8498 and λ 8542 A lines of the Ca II infrared triplet (IRT) have been obtained for a sample of late type dwarfs, with a wide range of levels of chromospheric activity, in the spectral range F9 to K4/5. We present these profiles, and the difference profiles for selected active and quiescent objects of corresponding spectral type. After absolute calibration in flux we take advantage of the high resolution and S/N ratios of the spectra to define several IRT indices of chromospheric activity, for a set of bandwidths between 0.2 A and 1 A about the line centres. Excellent correlations for active stars between the IRT and the Ca H indices and between the respective line widths are shown, which pertain despite the different depths of formation of the lines in the stellar chromospheres