Age Dependence of the Vertical Distribution of Young Open Clusters: Implications for Local Mass Density, Stellar Evolution, and Star Formation

Abstract

The ages of 203 open clusters from the list of Dambis (1999) are computed in terms of the evolutionary tracks of Pols et al. (1998) with and without the allowance for convective overshooting. The vertical scaleheight of the cluster layer at Galactocentric distances R_0-1 kpc < R_g< R_0+ 1 kpc varies nonmonotonically with age, exhibiting a wavelike pattern similar to the one earlier found for the Cepheid population (Joeveer 1974), with a period of P_Z = 74 +/- 2 Myr and P_Z = 92 +/- 2 Myr if cluster ages are computed in terms of evolutionary models without and with overshooting, respectively. The period of vertical oscillations can be reconciled with the known local mass density only if cluster ages are computed with no or just mild overshooting: the overshooting-based ages imply a local mass density of rho = 0.075 +/- 0.003 M_Sun/pc^3, which is incompatibe with the recent Hipparcos-based estimate of rho = 0.102 M_Sun/pc^3 (Holmberg and Flynn 2000). Our results imply a maximum local dark-mass density of rho_DM <= 0.027 M_Sun/pc^3. At the time of their formation open clusters have, on the average, excess vertical kinetic energy and as a population are not in virial equilibrium; moreover, their initial vertical coordinates (at the time of birth) are strongly and positively correlated with initial vertical velocities (r = 0.81 +/-0.08), thus favoring a scenario where star formation in the disk is triggered by some massive objects falling onto the Galactic plane.