ON THE USE OF [NA FE] AND [ALPHA/FE] RATIOS AND HIPPARCOS-BASED (U, V, W) VELOCITIES AS AGE INDICATORS AMONG LOW-METALLICITY HALO FIELD GIANTS/

We have examined the [Na/Fe] and [Mg/Fe] ratios in a sample of 68 held halo giants with -3 less than or similar to [Fe/H] less than or simil ar to -1. We recalculated the Galactic (U, V, W) velocity components f or these stars, using Hipparcos proper motions and a new Hipparcos-bas ed distance scale. We used these data to see how the abundance ratios may relate to kinematical substructure in the Galactic halo. To isolat e a set of true halo stars, we eliminated metal-weak thick-disk stars, about 10% of our sample. The field halo giants show the expected corr elation of Na and Mg abundances, so we can use Na as a surrogate for M g and the alpha-elements. The most metal-poor stars show a wider dispe rsion of [Na/Fe] ratios than do the less metal-poor stars; the differe nce is most striking for stars on retrograde galactic orbits. Some 20% of our retrograde giants and 13% of all our halo giants have [Na/Fe] less than or equal to -0.35 and may be significantly younger than the oldest halo objects. Halo giants considered ''young'' by this Na abund ance criterion show a preference for retrograde orbits. Giants in some globular clusters (e.g., M13) do not exhibit the Mg versus Na correla tion found among halo held giants. Instead, they have very large [Na/F e] ratios and widely scattered [Mg/Fe] ratios, probably induced by dee p mixing, which field halo giants apparently do not experience.