AAS Meeting #193 - Austin, Texas, January 1999
Session 67. Open Clusters
Display, Friday, January 8, 1999, 9:20am-6:30pm, Exhibit Hall 1

## [67.11] Age--Velocity Relation for HIPPARCOS {\em uvby} F Stars and a Blue Straggler Population in the Local Disk.\\ \\

A. Suchkov (STScI)

For a large sample of F stars with HIPPARCOS parallaxes and proper motions, we find a strong correlation between mean tangential velocity and age derived from the 1996 Yale isochrones. The relationship between age and velocity appears to be the same for different metallicity groups of stars. This is consistent with star formation occurring in a chemically inhomogeneous ISM throughout the lifetime of the disk. The mean metallicity of stars is found to increase toward younger ages. The data suggest that formation of stars of solar metallicity peaked about 3 to 4 Gyr ago. The age--velocity relation provides evidence for presence of a significant population of blue stragglers and unresolved binaries among F stars. In the color--magnitude diagram, this population manifests itself in the first place as blue stars above the loci of the MS turn-offs, with kinematics typical for an age much older than the isochrone age at the location of these stars.