Star Formation History in the Solar Vicinity
  The star formation history in the solar neighbourhood is inferred comparing a sample of field stars from the Hipparcos Catalog with synthetic CMDs. We considered separately the main sequence and the red giant region of the HR diagram. The criteria for our best solutions are based on the $\chi^{2}$ minimization of star distributions in selected zones of the HR diagram. Our analysis suggests that: a) the solutions are compatible with a Salpeter IMF and with {\sl a star formation rate increasing, in a broad sense, from the beginning to the present time}; b) the deduced volume mass densities and the corresponding absolute scale of the SFR solutions are strongly influenced by the initial mass function slope of low mass stars (below 0.5 Mo); c) the stellar evolutionary models are not completely adequate: in fact {\sl the theoretical ratio between the He-burning and MS star numbers is always a factor 1.5 greater than the observational value}. This fact could indicate the need of a more efficient overshoot in the evolutionary models, or a different mixing theory. 
Bertelli Gianpaolo (1,2) and Nasi Emma (3) ((1) CNR-GNA Roma (2) Dep.   of Astron. Padova (3) Astron. Obs. Padova)