Age Dependence of the Vertical Distribution of Young Open Clusters

Sternberg Astronomical Institute, Universitetskii pr. 13, Moscow 119992, Russia

Abstract

The ages of 203 open clusters from the list of Dambis (1999) are computed in terms of Cambridge evolutionary tracks with and without the allowance for convective overshooting (Pols et al. 1998). The vertical scaleheight of the cluster layer for 123 objects at Galactocentric distances kpc is found to vary non-monotonically with age exhibiting a wavelike pattern similar to the one earlier found for the Cepheid population (Joeveer 1974). The period of these variations is equal to Myr and Myr if cluster ages are computed in terms of evolutionary models of Pols et al. (1998) without and with overshooting, respectively. If interpreted as a manifestation of vertical virial oscillations, the implications of the pattern found are threefold: (1) the period of vertical oscillations can be reconciled with the known local density of visible matter only if cluster ages are computed with no or mild overshooting ( Myr), which implies a maximum local density of  pc^-3 compared to pc^-3 recently inferred from Hipparcos data (Holmberg & Flynn 2000), whereas the period derived from ages computed using models with overshooting ( Myr) implies a maximum local density of only pc^-3 and is thus totally incompatible with recent estimates; (2) there is not much room left for the dark matter ( pc^-3) in the Galactic disk near the solar Galactocentric distance, and (3) at the time of their formation open clusters have, on the average, excess kinetic energy (in the vertical direction) and as a population are not in virial equilibrium; moreover, the initial vertical coordinates of open clusters (at the time of their birth) are strongly and positively correlated with initial vertical velocities (the correlation coefficient is ), thus favoring a scenario where star formation in the disk is triggered by some massive objects falling to the Galactic plane.