Exploring Halo Substructure with Giant Stars. I. Survey Description and Calibration of the Photometric Search Technique

We have begun a survey of the structure of the Milky Way halo, as well as thehalos of other Local Group galaxies, as traced by their constituent giantstars. These giant stars are identified via large area, CCD photometriccampaigns. Here we present the basis for our photometric search method, whichrelies on the gravity sensitivity of the Mg I triplet + MgH features near 5150Angstroms in F-K stars, and which is sensed by the flux in the intermediateband DDO51 filter. To calibrate our (M-T_2, M-DDO51) diagram as a means todiscriminate field giant stars from nearby dwarfs, we utilize new photometry ofthe main sequences of the open clusters NGC 3680 and NGC 2477 and the red giantbranches of the clusters NGC 3680, Melotte 66 and omega Centauri, supplementedwith data on field stars, globular clusters and open clusters by Doug Geislerand collaborators. By combining the data on stars from different clusters, andby taking advantage of the wide abundance spread within omega Centauri, weverify the primary dependence of the M-DDO51 color on luminosity, anddemonstrate the secondary sensitivity to metallicity among giant stars. Ourempirical results are found to be generally consistent with those from analysisof synthetic spectra by Paltoglou & Bell [1994, MNRAS, 268, 793].Comment: 46 pages, 14 figures To appear in October 2000 issue of The Astronomical Journa