2015A&A...576A..28P

Currently clusters and association 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 during the first 20Myr of their development. We investigate the effect of the gas expulsion on the detectability of clusters and associations typical of the solar neighbourhood, where the star formation efficiency is <35%. The main focus is on the dependence on the initial cluster mass. N-body methods are used to determine the cluster and association dynamics after gas expulsion. We find that, even for low background densities, only clusters and associations with initial central surface densities exceeding a few 1000M_☉/pc² will be detected as clusters at ages 5Myr. Even the Orion nebula cluster, one of the most massive nearby clusters, would only be categorised as a small co-moving group with current methods after 5Myr of development. This means that cluster expansion leads to a selection effect - at ages of <1-2Myr the full range of clusters and associations is observed whereas at ages >4Myr only the most massive clusters are identified, while systems with initially M_c<3000M_☉ 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 method highly questionable. However, Gaia could provide the means to rectify this situation because it will be able to detect lower mass clusters.