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Session 19 - Old Stellar Populations in the Milky Way.
Invited session, Monday, June 09
North Main Hall A,
Proper motion catalogs are rich sources of high-velocity stars, and recent work on space motions and metallicities of such stars indicate that the metal-poor stars in the solar neighborhood may have originated in two very different regimes. Most may have formed during the early contraction stages of our Galaxy, as described by Eggen, Lynden-Bell, and Sandage in 1962. But a significant number show kinematical signs of having formed in other galaxies and merged into the Milky Way at a later time.
Proper motion catalogs are highly biased against low-velocity stars, but a new 1/V_max algorithm developed by L. Aguilar can be applied to such samples to correct for such biases. One particularly revelvant question is whether the thick disk population was a progenitor to the thin disk or, like some of the metal-poor halo, was an accretion event.
Finally, a number of very high-velocity stars with unusual [\alpha/Fe] abundance ratios have been found, suggesting a chemical ``signature" for some merger events, as well as a relic of the victims' nucleosynthesis histories.
