Star Formation History in the Solar Vicinity

The star formation history in the solar neighborhood is inferred by comparing a sample of field stars from the Hipparcos catalog with synthetic color-magnitude diagrams. We consider separately the main sequence and the red giant region of the H-R diagram. The criteria for our best solutions are based on the χ2 minimization of star distributions in selected zones of the H-R diagram. Our analysis suggests (1) that the solutions are compatible with a Salpeter initial mass function and with a star formation rate increasing in a broad sense from the beginning to the present time, (2) that the deduced volume-mass densities and the corresponding absolute scale of the star formation rate solutions are strongly influenced by the initial mass function slope of low-mass stars (below 0.5 M⊙), and (3) that the stellar evolutionary models are not completely adequate: in fact the theoretical ratio between the He-burning and MS star numbers is always a factor of 1.5 greater than the observational value. This fact could indicate the need for a more efficient overshoot in the evolutionary models or a different mixing theory.