Issue |
EAS Publications Series
Volume 10, 2003
JENAM 2002, Galactic & Stellar Dynamics
|
|
---|---|---|
Page(s) | 147 - 147 | |
DOI | https://doi.org/10.1051/eas:2003140 | |
Published online | 22 December 2003 |
C. Boily, P. Patsis, S. Portegies Zwart, R. Spurzem and C. Theis (eds)
EAS Publications Series, 10 (2003) 147
Age Dependence of the Vertical Distribution of Young Open Clusters
Sternberg Astronomical Institute, Universitetskii pr. 13, Moscow 119992, Russia
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.
© EAS, EDP Sciences, 2003