Understanding EROS2 observations toward the spiral arms within a
  classical Galactic model framework
  EROS has searched for microlensing toward four directions in the Galactic
plane away from the Galactic center. The interpretation of the catalog optical
depth is complicated by the spread of the source distance distribution. We
compare the EROS microlensing observations with Galactic models, tuned to fit
the EROS source catalogs, and take into account all observational data such as
the microlensing optical depth, the Einstein crossing durations, and the color
and magnitude distributions of the catalogued stars. We simulated EROS-like
source catalogs using the Hipparcos database, the Galactic mass distribution,
and an interstellar extinction table. Taking into account the EROS star
detection efficiency, we were able to produce simulated color-magnitude
diagrams that fit the observed diagrams. This allows us to estimate average
microlensing optical depths and event durations that are directly comparable
with the measured values. Both the Besancon model and our Galactic model allow
us to fully understand the EROS color-magnitude data. The average optical
depths and mean event durations calculated from these models are in reasonable
agreement with the observations; consequently our simulation allows a better
understanding of the lens and source spatial distributions in the microlensing
events. Varying the Galactic structure parameters through simulation, we were
also able to deduce contraints on the kinematics of the disk, the disk stellar
mass function (IMF at a few kpc distance from the Sun), and the maximum
contribution of a thick disk of compact objects in the Galactic plane (Mthick <
5 - 7 x 10**10 Msun at 95%, depending on the model). We also show that the
microlensing data toward one of our monitored directions are significantly
sensitive to the galactic bar parameters, although much larger statistics are
needed to provide competitive constraints.
