The Besancon Galaxy model renewed I. Constraints on the local star
  formation history from Tycho data
  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. The new version of the Besancon Galaxy model (hereafter 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. 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. 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 Mo/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. 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.
