3 The performance of the 8500-8750 Å region in the GAIA context

The principal reason to have a spectrograph of suitable resolution aboard the GAIA astrometric mission is to provide the 6$^{\rm th}$ component of the phase-space coordinates. GAIA will observe in scanning-mode, therefore all targets will be observed for the same exposure time irrespective of their magnitude. Given the brightness population index for field stars (an increase of 1 in the limiting magnitude roughly triples the number of stars), the vast majority of GAIA targets will be found among the stars providing the weakest signal above a given threshold.

The weaker the spectrum, the stronger must be the spectral features to maintain a reasonable accuracy in the radial velocities (cf. M 99). The grid of spectra presented in this paper show how in the whole near-IR region explored (7650-8750 Å), the Ca II triplet is by far the strongest feature for the spectral types, luminosity classes and metal abundances accounting for the vast majority of the GAIA targets.

Therefore, the present atlas adds support to the superior performance of the 8500-8750 Å region in meeting the GAIA demands on radial velocities (cf. M 99 for a discussion about the performances in terms of rotational velocities, spectral classification, chemical abundances, detection of mass loss and peculiarities, signatures of interstellar reddening).