Absorption cells in wavelength calibration of GAIA spectra

The wavelength calibration of the GAIA spectra is a crucial point in deriving accurate radial velocity for the program stars. The scanning slit-less mode does not allow to use the usual wavelength calibration techniques. Current plans foresee to calibrate the spectra by the identification of a suitable ensemble of reference stars, that would map the wavelength solution in the different position of the focal plane. We discuss an alternative solution, the use of an absorption cell that imprints its own lines on top of each spectrum, allowing an accurate wavelength calibration. The attanaible precision is discussed in analytical form. Absorption cells were widely used in recent years mainly for the radial velocity searches of extra-solar planets. The major issue for the use of absorption cells within the GAIA spectrograph is represented by the identification of a suitable absorbing medium that produce a few lines in the spectral region of interest with a small optical path and, more crucially, at the satellite operating temperature. Possible media are discussed and tests at the telescope anticipated.