On the Use of [Na/Fe] and [α/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 field halo giants with -3 ≲ [Fe/H] ≲ -1. We recalculated the Galactic (U, V, W) velocity components for these stars, using Hipparcos proper motions and a new Hipparcos-based distance scale. We used these data to see how the abundance ratios may relate to kinematical substructure in the Galactic halo. To isolate 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 correlation of Na and Mg abundances, so we can use Na as a surrogate for Mg and the α-elements. The most metal-poor stars show a wider dispersion of [Na/Fe] ratios than do the less metal-poor stars; the difference 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] ≤ -0.35 and may be significantly younger than the oldest halo objects. Halo giants considered "young" by this Na abundance criterion show a preference for retrograde orbits. Giants in some globular clusters (e.g., M13) do not exhibit the Mg versus Na correlation found among halo field giants. Instead, they have very large [Na/Fe] ratios and widely scattered [Mg/Fe] ratios, probably induced by deep mixing, which field halo giants apparently do not experience.