AAS 199th meeting, Washington, DC, January 2002
Session 91. Stellar Populations and Galactic Structure
Display, Wednesday, January 9, 2002, 9:20am-6:30pm, Exhibit Hall

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[91.08] The HK-II Survey: A New Search for Cooler, Extremely Metal-Deficient Stars in the Milky Way Galaxy

J. Rhee (U. of Virginia & Michigan State U.), T.C. Beers (Michigan State U.), M.J. Irwin (Institute of Astronomy, Cambridge, UK)

We have recently completed digital scans of the original HK survey (HK-I) plates using the Automatic Plate Measuring machine, and developed a new metal-poor star selection methodology which makes use of Artificial Neural Network techniques to analyse some 1.5 million HK digital spectra in combination with available 2MASS JHK photometry (the HK-II Survey). Of primary importance, the objective selection criteria employed in the HK-II survey avoids the introduction of temperature-related bias in the identification of metal-deficient stars, a difficulty which limited the identification of cooler metal-poor stars with HK-I. Preliminary tests with known metal-poor stars have revealed that our new approach should result in a detection efficiency of roughly 60-70% for stars predicted to have [Fe/H] \le -2.0, three times higher than obtained by HK-I. Recent spectroscopic observations support this higher detection rate of bona-fide metal-deficient giant stars among HK-II candidates. >From the HK-II survey we expect to obtain a new database of some 5000 cooler metal-poor candidates, consisting mainly of giant stars, over ~7000 deg2 in the thick disk and halo of the Milky Way Galaxy (to date, some 2000 candidates have already been selected).

Metallicities and radial velocities determined from medium-resolution spectroscopic follow-up, combined with proper motions from ongoing and future astrometric surveys will provide full space motions for numerous extremely old stars. The results will help unravel the chemical and dynamical history of the Milky Way. The newly identified metal-poor giant stars will aid us in obtaining a more reliable metallicity distribution function of the halo, especially the metal-weak tail, and provide a large sample of giant candidates to be searched for stars with highly enhanced r-process elements by high-resolution spectroscopic follow-up. The HK-II giant catalog will also provide an independent check on the distances and metallicities obtained for Astrometric Grid K giant candidates for NASA's SIM mission.

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