On the Use of [N[CLC]a[/CLC]/F[CLC]e[/CLC]] and [α/F[CLC]e[/CLC]] Ratios and [ITAL]Hipparcos[/ITAL]-based ([ITAL]U[/ITAL], [ITAL]V[/ITAL], [ITAL]W[/ITAL]) 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 Ðeld halo giants with We recalculated the Galactic (U, V , W ) velocity components for these stars, usingHipparcos 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 Ðeld halo giants show the expected correlation of Na and Mg abundances, so we can use Na as a surrogate for Mg and the a-elements.The most metal-poor stars show a wider dispersion of [Na/Fe] ratios than do the less metal-poor stars ; the di †erence 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] ¹ [0.35 and may be signiÐcantly younger than the oldest halo objects.Halo giants considered "" young ÏÏ 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 Ðeld giants.Instead, they have very large [Na/Fe] ratios and widely scattered [Mg/Fe] ratios, probably induced by deep mixing, which Ðeld halo giants apparently do not experience.