For several metallic ions there is in this region the contemporaneous presence of absorption lines from the same multiplet as well as from multiplets of largely different excitation potential (cf. Table 4). This offers a bright prospect for a quantitative spectral classification. Later on in this series line ratios useful for a quantitative classification will be investigated (work based on extensive measurement of observational and Kurucz's synthetic spectra).
The actual stellar spectra are obviously continously distributed in temperature and luminosity even if all past and present classification systems have forced them into discrete bins (only qualitatively defined by a set of standards to compare with). With an expected harvest of some 107 extremely homogeneous spectra, GAIA is perfectly suited to move forward the classification process into a continous and fully quantitative scheme. For all the spectroscopic targets GAIA will provide accurate astrometric distances, color temperature from multiband photometry and metallicities from spectroscopy. It should be therefore easy to derive an accurate and continous classification in terms of temperature, luminosity and metallicity from the equivalent width ratios of appropriately chosen absorption lines.
Acknowledgements
The encouragement by F. Favata, M. Perryman (ESA), P.L. Bernacca (Asiago), M.G. Lattanzi (Torino) as well as discussions with C. Chiosi (Padova), K. de Boer (Bonn), T.V. Tomov (Asiago) and H. Lamers (Utrecht) have been useful to this program. F. Castelli (Trieste) provided for the hottest stars synthetic spectra useful to check the continuum normalization procedure. The present investigation has been partially supported by ASI (Italian Space Agency) contract ARS-96-175 to CISAS, Univ. of Padova. We have greatly benefitted from extensive access to the SIMBAD database at CDS in Strasbourg.
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