Revisiting the population of Galactic open clusters

Abstract

We present results of a study of the galactic open cluster population based on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2, Hipparcos and other catalogues. The sample of optical clusters from ASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry plane of the clusters' distribution is determined to be at pc, and the scale height of open clusters is only pc. The total surface density and volume density in the symmetry plane are kpc-2 and kpc-3, respectively. We find the total number of open clusters in the Galactic disk to be of order of 105 at present. Fluctuations in the spatial and velocity distributions are attributed to the existence of four open cluster complexes (OCCs) of different ages containing up to a few tens of clusters. Members in an OCC show the same kinematic behaviour, and a narrow age spread. We find, that the youngest cluster complex, OCC 1 (), with 19 deg inclination to the Galactic plane, is apparently a signature of Gould's Belt. The most abundant OCC 2 complex has moderate age (). The clusters of the Perseus-Auriga group, having the same age as OCC 2, but different kinematics are seen in breaks between Perseus-Auriga clouds. The oldest () and sparsest group was identified due to a large motion in the Galactic anticentre direction. Formation rate and lifetime of open clusters are found to be kpc-2 Myr-1 and Myr, respectively. This implies a total number of cluster generations in the history of the Galaxy between 30 to 40. We estimate that less than about 10% of the total Galactic stellar disk population has ever passed an open cluster membership.