A MODEL OF STELLAR EVOLUTION OF THE GALACTIC DISC - THE IMF AT LOW MASSES

This work is part of a program undertaken to study the evolution of ou r Galaxy using star-counts in several directions. Thanks to the recent advances in data processing, important sets of star-count data are no w becoming available for the study of the galactic structure and the s tellar populations in the Milky Way. Recent progress in the stellar ev olution theory also suggest that galactic evolution studies should tak e advantage of this observational material. Star-count data include ma gnitude, colours and proper motions, but give no direct information on fundamental parameters such as distance, mass, or metallicity. Theref ore, our ability to interpret these data rely on models of stellar pop ulation synthesis, which have been actively developed in the last deca de. In most of these models the color-magnitude diagram for the disc p opulation is fixed, and can give no information about the initial mass function and star formation rate (hereafter IMF and SFR). We describe here how such distribution for our galactic disc is computed for a gi ven SFR and IMF, give the inputs of the model and take this occasion t o investigate the solar neighbourhood luminosity function at magnitude s fainter than M(v) almost-equal-to 5. We discuss the slope of the IMF in the range [0.1, 1] solar mass. The consequences on the IMF of the Wielen dip feature and the decrease - or flattening - of the luminosit y function at M(v)=12 are discussed. Analysis of star-count data and r esults concerning the galactic disc evolution will be presented in a f uture paper.