Improved color-temperature relations and bolometric corrections for cool stars

We present new grids of colors and bolometric corrections for F-K stars having 4000 K less than or equal to T-eff less than or equal to 6500 K, 0.0 less than or equal to log g less than or equal to 4.5, and -3.0 less than or equal to [Fe/H] less than or equal to 0.0. A companion paper extends these calculations into the M giant regime (3000 K less than or equal to T-eff less than or equal to 4000 K). Colors are tabulated for Johnson U - V and B - V, Cousins V - R and V - I, Johnson-Glass V - K, J - K, and H - K, and CIT/CTIO V - K, J - K, H - K, and CO. We have developed these color-temperature relations by convolving synthetic spectra with the best-determined, photometric filter transmission profiles. The synthetic spectra have been computed with the SSC spectral synthesis code using MARCS stellar atmosphere models as input. Both of these codes have been improved substantially, especially at low temperatures, through the incorporation of new opacity data. The resulting synthetic colors have been put onto the observational systems by applying color calibrations derived from models and photometry of field stars that have effective temperatures determined by the infrared flux method. These color calibrations have zero points that change most of the original synthetic colors by less than 0.02 mag, and the corresponding slopes generally alter the colors by less than 5%. The adopted temperature scale, that of Bell & Gustafsson, is confirmed by the extraordinary agreement between the predicted and observed angular diameters of these field stars, indicating that the differences between the synthetic colors and the photometry of the field stars are not due to errors in the effective temperatures adopted for these stars. Thus, we have derived empirical color-temperature relations from the field star photometry, which we use as one test of our calibrated, theoretical, solar-metallicity color-temperature relations. Except for the coolest dwarfs (T-eff < 5000 K), our calibrated model colors are found to match these relations, as well as the empirical relations of others, quite well, and our calibrated, 4 Gyr, solar-metallicity isochrone also provides a good match to color-magnitude diagrams of M67. We regard this as evidence that our calibrated colors can be applied to many astrophysical problems, including modeling the integrated light of galaxies. Because there are indications that the dwarfs cooler than 5000 K may require different optical color calibrations than the other stars, we present additional colors for our coolest dwarf models that account for this possibility.