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 the halos of other Local Group galaxies, as traced by their constituent giant stars. These giant stars are identified via large-area, CCD photometric campaigns. Here we present the basis for our photometric search method, which relies on the gravity sensitivity of the Mg I triplet+MgH features near 5150 Angstrom in F-K stars, and which is sensed by the flux in the intermediate-band DDO51 filter. Our technique is a simplified variant of the combined Washington/DDO51 four-filter technique described by Geisler, which we modify for the specific purpose of efficiently identifying distant giant stars for follow-up spectroscopic study: We show here that for most stars the Washington 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 correlation obviates the need to image in the T-1 filter, as originally proposed by Geisler. To calibrate our (M-T-2, M-DDO51) diagram as a means to discriminate held giant stars from nearby dwarfs, we utilize new photometry of the main sequences of the open clusters NGC 3680 and NGC 2477 and the red giant branches of the clusters NGC 3680, Melotte 66, and omega Centauri, supplemented with data on field stars, globular clusters and open clusters by Doug Geisler and collaborators. By combining the data on stars from different clusters, and by taking advantage of the wide abundance spread within omega Centauri, we verify the primary dependence of the M - DDO51 color on luminosity and demonstrate the secondary sensitivity to metallicity among giant stars. Our empirical results are found to be generally consistent with those from analysis 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 giant branch curves from omega Centauri stars for use in deriving photometric parallaxes for giant stars of various metallicities (but equivalent ages) to those of omega Centauri giants.