We use the Hipparcos survey to derive an
improved model of the local galactic structure.
The availability of parallaxes for all the stars
permits direct determination of stellar
distributions, eliminating the basic indeterminacy
of classical methods based on star counts.
Hipparcos gives for the first time a truly
three-dimensional view of the solar vicinity, and
a complete, homogeneous and highly accurate set of
magnitudes and colours. This means that new
techniques can be applied in the treatment of the
data which place strong constraints on a model
that tries to describe the local Galactic
structure. Here we investigate how well a static
model of low complexitycan describe the Hipparcos
observations.
The interpretation of the Hipparcos data
is complicated by various observational errors and
selection effects that are hard to treat
correctly. We do not try to correct the data, but
instead use a model and subject this model to the
same observational errors and selection effects. A
model catalogue is created that can be compared
with the observed catalogue directly in the
observational domain, thereby eliminating the
effects from various biases.
Many features in the HR diagram are for
the first time seen in field stars thanks to
Hipparcos, such as the slanted red giant clump,
previously seen in rich old open clusters such as
Berkeley 18. This and other features ofthe
observed HR diagram are well reproduced by the
model thanks to the rather detailed modelling of
the joint Mv/B — V
distribution. Actually, separate distributions
were derived for the three different components,
disk, thick disk and halo, using the kinematic
characteristics of the components to discriminate
between them.
Part IV. Galactic Structure and Explosion of the Galactic Nucleus
