AAS 201st Meeting, January, 2003
Session 74. Stars: Observational Programs
Poster, Wednesday, January 8, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[74.01] The Grid Giant Star Survey (GGSS) for SIM: A Status Report

C.B. Hummels, J. Rhee, A.A. Polak, J.D. Crane, R.J. Patterson, S.R. Majewski (UVa), W.E. Kunkel (LCO), D. Geisler, J. Seguel, W. Gieren (U. Concepción), C.L. Slesnick (Caltech), A. Kundu (MSU), G.F. Benedict (U. Texas, Austin), K.V. Johnston, C. Moskowitz (Wesleyan)

To perform wide-angle astrometry of stars to an accuracy of 4 \muas, NASA's Space Interferometry Mission (SIM) requires an astrometrically stable reference frame composed of relatively bright stars (V<13), the ``Astrometric Grid''. Low metal-abundance K giants are good candidate stars for the SIM Astrometric Grid because they are the most distant stars brighter than V=13 that are also common enough to fill an all-sky grid pattern. Large distances minimize astrometric jitter due to planets, binary stars, starspots, etc. The goal of the GGSS is to identify the most distant metal-poor K giants in 1302 ``bricks'' distributed over the entire sky. The ~.5 deg{}2 bricks have a mean separation of ~\circ. Photometric observations with the Washington M, T2 and the (gravity sensitive) DDO51 filters have been completed in 99% of these bricks, using the 1-m Swope telescope at Las Campanas Observatory, the 0.8-m telescope at McDonald Observatory, and the WIYN 0.9-m telescope. A semi-automated reduction pipeline generates a list of giant candidates in each brick, along with photometric parallaxes and metallicities derived from the three-filter system. In each brick, the (up to) four most distant giant candidates with M<13.5 are selected as Grid candidates. To date, 755 bricks (58%) have been completely reduced, and 96% of these fields contain at least one candidate with a photometric distance > 1 kpc. The best SIM candidate for each brick has a median distance of ~4.0 kpc and metallicity of [Fe/H] ~1.0. At present, we have obtained spectra of over 2800 grid candidates in the wavelength range 4800 - 6800 Å\ using the Modular Spectrograph on the Swope telescope. This enables us to verify the luminosity class and determine abundances and radial velocities of the candidate giants. Among these spectra we find an interesting subsample of bright but very metal poor ([Fe/H] < -2.5) stars. Here we present a status report and other results from the GGSS.

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