The Besançon Galaxy Model renewed. I. Constraints on the Galactic thin disc evolution from Tycho data

Context. The understanding of Galaxy evolution can be facilitated by the use of population synthesis models, which allows us to test hypotheses on the star formation history, star evolution, and chemical and dynamical evolution of the Galaxy. Aims. The new version of the Besancon Galaxy model (BGM) aims to provide a more flexible and powerful tool to investigate the initial mass function (IMF) and star formation rate (SFR) of the Galactic disc. Methods. We present a new strategy for the generation of thin disc stars, which assumes the IMF, SFR and evolutionary tracks as free parameters. We have updated most of the ingredients for the star count production and, for the first time, binary stars are generated in a consistent way. The local dynamical self-consistency is maintained in this new scheme. We then compare simulations from the new model with Tycho-2 data and the local luminosity function, as a first test to verify and constrain the new ingredients. The effects of changing thirteen different ingredients of the model are systematically studied. Results. For the first time, a full sky comparison is performed between BGM and data. This strategy allows us to constrain the IMF slope at high masses, which is found to be close to 3.0 and excludes a shallower slope such as Salpeter's one. The SFR is found decreasing whatever IMF is assumed. The model is compatible with a local dark matter density of 0.011 M-circle dot pc(-3) implying that there is no compelling evidence for the significant amount of dark matter in the disc. While the model is fitted to Tycho-2 data, which is a magnitude limited sample with V < 11, we check that it is still consistent with fainter stars. Conclusions. The new model constitutes a new basis for further comparisons with large scale surveys and is being prepared to become a powerful tool for the analysis of the Gaia mission data.