Empirically constrained color-temperature relations.: I.: BV(RI)C

This investigation presents a set of transformations to Johnson B-V, Cousins V-R, and Cousins V-I, as well as bolometric corrections to V, for [Fe/H] = -3, -2, -1, -0.5, 0.0, and +0.3 and, in each case, values of log g from -0.5 to 5.0 for 3000 K less than or equal to T-eff less than or equal to 5500 K and from 2.0 to 5.0 for 6000 K less than or equal to T(ef)f 40,000 K. These transformations employ the predictions from Kurucz model atmospheres at high temperatures (T-eff greater than or equal to 8000 K) and from MARCS model atmospheres at intermediate temperatures (from 7000 K down to a temperature in the range 4000 K less than or equal to T-eff less than or equal to 5500 K, depending on [Fe/H], where adjustments to satisfy observational constraints become necessary). Thus, theoretical color-T-eff relations are used exclusively down to a minimum temperature that is cooler than the temperatures of turnoff stars in open and globular star clusters. To better represent the color transformations obeyed by cool stars (down to 3000 K), corrections to the synthetic transformations have been determined from a careful consideration of observations for a few globular clusters (M92, M68, and 47 Tucanae), the color-magnitude diagrams (CMDs) of several open clusters (M67, the Pleiades, the Hyades, and NGC 6791), the CMDs and mass-luminosity diagram for solar neighborhood stars having good distance measurements from Hipparcos, empirical (B-V)-T-eff and (V-K)-T-eff relations, and color-color diagrams for field giants. The semiempirical color transformations that have been produced as a result of our analysis are also compared with several others that have been published in recent years: some of the deficiencies of the latter are revealed.