Basic calibrations of the photographic RGU system IV. Metal-poor subgiant and giant stars

We present a large grid of theoretical color-absolute magnitude and two-color diagrams in the RGU photometric system. The grid comprises wide ranges of stellar ages and physical parameters, and was computed using the YALE isochrones (Demarque et al. 1996) combined with the most recent version of the Basel Stellar Library (hereafter BaSeL) of color-calibrated theoretical model-atmosphere flux distributions (Lejeune et al. 2000). The present data provide a significant expansion of vital calibrations which are required for the full analysis of the new Basel three-color high-latitude field star survey of the Galaxy (Buser et al. 1998). In particular, they bring about, for the first time, the consistent determination of metallicities and metallicity-dependent absolute magnitudes for subgiant and giant stars from observed ultraviolet excesses, delta(U - G)(G - R), and G - R colors, respectively.