Standard Giant Branches in the Washington Photometric System

Abstract

We have obtained CCD photometry in the Washington system C, T_1 filters for some 850,000 objects associated with 10 Galactic globular clusters and two old open clusters. These clusters have well-known metal abundances, spanning a metallicity range of 2.5 dex from [Fe/H]~-2.25 to +0.25 at a spacing of ~0.2 dex. Two independent observations were obtained for each cluster, and internal checks, as well as external comparisons with existing photoelectric photometry, indicate that the final colors and magnitudes have overall uncertainties of ~5 Gyr-any age effects are small or negligible for such objects. This new technique is found to have many advantages over the previous two-color diagram technique for deriving metallicities from Washington photometry. In addition to requiring only two filters instead of three or four, the new technique is generally much less sensitive to reddening and photometric errors, and the metallicity sensitivity is many times higher. The new technique is especially advantageous for metal-poor objects. The five metal-poor clusters determined by Geisler et al., using the old technique, to be much more metal-poor than previous indications, yield metallicities using the new technique that are in excellent agreement with the Zinn scale. The anomalously low metallicities derived previously are undoubtedly a result of the reduced metallicity sensitivity of the old technique at low abundance. However, the old technique is still competitive for metal-rich objects ([Fe/H]>~-1). We have extended the method developed by Sarajedini to derive simultaneous reddening and metallicity determinations from the shape of the red giant branch (RGB), the T_1 magnitude of the horizontal branch, and the apparent (C-T_1) color of the RGB at the level of the horizontal branch. This technique allows us to measure reddening to 0.025 magnitudes in E(B-V) and metallicity to 0.15 dex. Reddenings can also be derived from the blue edge of the instability strip, with a similar error. We measure the apparent T_1 magnitude of the red giant branch bump in each of the calibrating clusters and find that the difference in magnitude between the bump and the horizontal branch is tightly and sensitively correlated with metallicity, with an rms dispersion of 0.1 dex. This feature can therefore also be used to derive metallicity in suitable objects. Metallicity can be determined as well from the slope of the RGB, to a similar accuracy. Our very populous color-magnitude diagrams reveal the asymptotic giant branch bump in several clusters. Although M_T_1 of the RGB tip is not as constant with metallicity and age as M_I, it is still found to be a useful distance indicator for objects with [Fe/H]