Young, High-Velocity A Stars. II. Misidentified, Ejected, or Unique?

Rodgers, Harding, and Sadler identified a group of A stars at the south Galactic (SGP) pole with properties unlike those of any stellar population. They found the stars to be at distances from the disk of 1 kpc to more than 4 kpc, with a velocity dispersion perpendicular to the plane of 66 km s^-1, yet they appear to be young stars. In this study, the ages, abundances, and kinematics of a large number of early-type stars at the SGP have been derived in order to examine the properties and augment the sample of high-velocity stars. In striking contrast to a comparative group of normal A stars near the disk the high-velocity A stars were all formed within the last 6.5×10⁸yr. Their calcium abundances range from -0.5 dex to 0.0 dex, and their W velocity dispersion is 62 km s^-1. It is shown that the stars are not misidentified horizontal-branch stars. It is suggested that at around 6.5×10⁸yr ago, a major source of relatively low abundance hydrogen was accreted by the Galactic disk, forming young high-velocity stars that do not partake of the age-abundance-kinematics relationships shown by other stellar groupings.