Empirical calibration of the near-infrared CaII triplet - I. The stellar library and index definition

A new stellar library at the near-IR spectral region developed for the empi rical calibration of the Ca II triplet and stellar population synthesis mod elling is presented. The library covers the range lambda lambda 8348-9020 a t 1.5-Angstrom (FWHM) spectral resolution, and consists of 706 stars spanni ng a wide range in atmospheric parameters. We have defined a new set of nea r-IR indices, CaT*, CaT and PaT, which mostly overcome the limitations of p revious definitions, the former being specially suited for the measurement of the Ca II triplet strength corrected for the contamination from Paschen lines. We also present a comparative study of the new and the previous Ca i ndices, as well as the corresponding transformations between the different systems. A thorough analysis of the sources of index errors and the procedu re to calculate them is given. Finally, index and error measurements for th e whole stellar library are provided together with the final spectra.