Which young clusters/associations are we missing today?
  Currently clusters/associations of stars are mainly detected as surface
density enhancements relative to the background field. While clusters form,
their surface density increases. It likely decreases again at the end of the
star formation process when the system expands as a consequence of gas
expulsion. Therefore the surface density of a single cluster can change
considerably in young clusters/associations during the first 20 Myr of their
development. We investigate the effect of the gas expulsion on the
detectability of clusters/associations typical for the solar neighborhood,
where the star formation efficiency is <35%. The main focus will be laid on the
dependence on the initial cluster mass. Nbody methods are used to determine the
cluster/association dynamics after gas expulsion. We find that, even for low
background densities, only clusters/associations with initial central surface
densities exceeding a few 5000 M(sun)/pc2 will be detected as clusters at ages
~5 Myr. Even the Orion Nebula cluster, one of the most massive nearby clusters,
would only be categorized as a small co-moving group with current methods after
5 Myr of development. This means that cluster expansion leads to a selection
effect - at ages of <1-2 Myr the full range of clusters/associations is
observed whereas at ages > 4 Myr only the most massive clusters are identified,
while systems with initially M_c < 3 000 M(sun) are missing. The temporal
development of stellar properties is usually determined by observing clusters
of different ages. The potentially strong inhomogeneity of the cluster sample
makes this methods highly questionable. However, GAIA could provide the means
to rectify this situation as it will be able to detect lower mass clusters.
