Young open clusters space-age distribution

Based on published photoelectric and CCD photometry and using the empirical ZAMS of Kholopov (1980, AZh, 57, 12) and the evolutionary-deviation curves obtained from the grid of isochrones computed by Maeder and Meynet (1991A&AS...89..451M), we determine the distances, color excesses, and ages for 203 Galactic open clusters with logt<8.2. The validity of the derived distance scale is corroborated by the Hipparcos trigonometric parallaxes of seven clusters.

The age distribution of the clusters is distorted by observational selection, which is enhanced with increasing heliocentric distance and which is virtually independent of interstellar extinction. As a result, the youngest (logt<=7.5) clusters are represented almost exclusively at heliocentric distances >2kpc, and the mean age of the cataloged clusters with logt<=100Myr decreases from 50Myr in the solar neighborhood down to 15Myr at heliocentric distances >2.4kpc.