Revisiting the population of Galactic open clusters

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 Z 0 = -22 ± 4 pc, and the scale height of open clusters is only 56 ± 3 pc. The total surface density and volume density in the symmetry plane are Σ = 114 kpc -2 and D(Z 0 ) = 1015 kpc -3 , respectively. We find the total number of open clusters in the Galactic disk to be of order of 10 5 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 (log t < 7.9), 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 (log t 8.45). 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 (log t 8.85) 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 0.23 ± 0.03 kpc -2 Myr -1 and 322 ± 31 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.