Age Dependence of the Vertical Distributionof Young Open Clusters

Abstract

The ages of 203 open clusters from the list of Dambis(1999) are computed in terms of Cambridge evolutionarytracks with and without the allowance for convective overshooting(Pols et al. 1998). The vertical scaleheight of thecluster layer for 123 objects at Galactocentric distances$ R_{\rm g} =R_0 \pm1$ kpc is found to varynon-monotonically with age exhibiting a wavelike pattern similarto the one earlier found for the Cepheid population (Joeveer1974). The period of these variations is equal to $P_{Z}= 74 \pm 2$ Myr and $P_{Z} = 92 \pm 2$ Myr if cluster ages arecomputed in terms of evolutionary models of Pols et al. (1998) without and with overshooting, respectively. Ifinterpreted as a manifestation of vertical virial oscillations,the implications of the pattern found are threefold: (1) theperiod of vertical oscillations can be reconciled with the knownlocal density of visible matter only if cluster ages are computedwith no or mild overshooting ($P_{Z} = 74 \pm 2$ Myr), which impliesa maximum local density of $\rho = 0.118 \pm 0.006~M_{\odot}$ pc^-3 compared to $\rho = 0.102~M_{\odot}$ pc^-3recently inferred from Hipparcos data (Holmberg & Flynn2000), whereas the period derived from ages computed usingmodels with overshooting ($P_{Z} = 92 \pm 2$ Myr) implies amaximum local density of only $\rho = 0.075 \pm 0.003~M_{\odot}$ pc^-3 and is thus totally incompatible with recentestimates; (2) there is not much room left for the dark matter($\rho_{\rm DM} \le 0.027~M_{\odot}$ pc^-3) in the Galactic disknear the solar Galactocentric distance, and (3) at the time oftheir formation open clusters have, on the average, excesskinetic energy (in the vertical direction) and as a populationare not in virial equilibrium; moreover, the initial verticalcoordinates of open clusters (at the time of their birth) arestrongly and positively correlated with initial verticalvelocities (the correlation coefficient is $r(Z_0,V_{Z(0)}) =0.81 \pm 0.08$), thus favoring a scenario where star formation inthe disk is triggered by some massive objects falling to theGalactic plane.