KINEMATIC AGES OF OB ASSOCIATIONS

Astrometric data from the HIPPARCOS mission include measurements of ea rly-type stars in nearly all OB associations within 1 kpc from the Sun , and are available as of 1996. In anticipation thereof we studied the determination of kinematic ages and initial sizes of OB associations, These studies have traditionally been done using proper motion data. We investigate their reliability by generating synthetic data, using s elf-consistent N-body simulations of an OB association. We consider tw o classical methods for deriving the kinematic age. First, the proper motions of the stars are traced back in time to the smallest configura tion in the past, which is assumed to correspond to the initial state of the association. Secondly, the proper motion in a certain direction is plotted versus the corresponding coordinate, and then the linear e xpansion coefficient is measured to derive the kinematic age. We find that the first method always leads to underestimated ages. All age est imates converge to similar to 4 Myr. The second method can lead to ove restimated as well as underestimated ages, depending on the chosen coo rdinate direction and the magnitude of the effects of virtual expansio n caused by radial motion. The first method also provides an estimate of the initial size of the OB association, which is always overestimat ed. We conclude that the longstanding discrepancy between the kinemati c and nuclear ages for OB associations can be attributed to underestim ates of the kinematic age.