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1998AJ....116.1286H - Astron. J., 116, 1286-1294 (1998/September-0)

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.

HANSON R.B., SNEDEN C., KRAFT R.P. and FULBRIGHT J.

Abstract (from CDS):

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.