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 t he halos of other Local Group galaxies, as traced by their constituent gian t stars. These giant stars are identified via large-area, CCD photometric c ampaigns. Here we present the basis for our photometric search method, whic h relies on the gravity sensitivity of the Mg I triplet+MgH features near 5 150 Angstrom in F-K stars, and which is sensed by the flux in the intermedi ate-band DDO51 filter. Our technique is a simplified variant of the combine d Washington/DDO51 four-filter technique described by Geisler, which we mod ify for the specific purpose of efficiently identifying distant giant stars for follow-up spectroscopic study: We show here that for most stars the Wa shington T-1-T-2 color is correlated monotonically with the Washington M-T, color with relatively low scatter; for the purposes of our survey, this co rrelation obviates the need to image in the T-1 filter, as originally propo sed by Geisler. To calibrate our (M-T-2, M-DDO51) diagram as a means to discriminate held g iant stars from nearby dwarfs, we utilize new photometry of the main sequen ces of the open clusters NGC 3680 and NGC 2477 and the red giant branches o f the clusters NGC 3680, Melotte 66, and omega Centauri, supplemented with data on field stars, globular clusters and open clusters by Doug Geisler an d collaborators. By combining the data on stars from different clusters, an d by taking advantage of the wide abundance spread within omega Centauri, w e verify the primary dependence of the M - DDO51 color on luminosity and de monstrate the secondary sensitivity to metallicity among giant stars. Our e mpirical results are found to be generally consistent with those from analy sis of synthetic spectra by Paltoglou & Bell. Finally, we provide conversion formulae from the (M, M-T-2) system to the ( V, V-I) system, corresponding reddening laws, as well as empirical red gian t branch curves from omega Centauri stars for use in deriving photometric p arallaxes for giant stars of various metallicities (but equivalent ages) to those of omega Centauri giants.