We present new grids of colors and bolometric corrections for F-K stars having 4000 K ≤ T eff ≤ 6500 K, 0.0 ≤ log g ≤ 4.5 and -3.0 ≤ [Fe/H] ≤ 0.0. A companion paper extends these calculations into the M giant regime (3000 K ≤ T eff ≤ 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 SSG 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 which have effective temperatures determined by the infrared-flux method. These color calibrations have zero points which 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%. 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 modelling 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 which account for this possibility.

The FOCAP multi‐object fibre optic system on the AAT has been used to obtain spectra for a sample of more than a hundred faint stars in the globular cluster 47 Tuc (NGC 104), from the base of the giant branch to about a magnitude below the main‐sequence turn‐off. The spectra cover the ultraviolet and blue bands of CN and the G band of CH. Quantitative abundances have been derived by comparing the observations with synthetic spectra. The main‐sequence stars can be divided into two approximately equal groups, one having N enhanced by a factor of ∼ 7, and C depleted by 40 per cent, relative to the other, similar to what is inferred for the red giants. The mean heavy‐element abundance, as measured by Ca or Fe, differs between the two groups by no more than 0.01 ± 0.05 dex.  All the available abundance data indicate that the atmospheres of 47 Tuc stars contain different amounts of material which has been processed through the CNO cycle, but not through more advanced stages of nucleosynthesis. In the red giants, this could be a result of the convective dredge‐up of processed material from deep within the stars themselves, as occurs in some more metal‐poor clusters. However, stars on the upper main sequence of globular clusters do not have convective envelopes, while any deep mixing there would have consequences incompatible with other observational data. Thus it seems that the main‐sequence C and N variations must be either primordial or caused by a pollution mechanism. The similarity of the bimodal abundance patterns among the dwarfs and giants then suggests that the dredge up of CNO‐cycled material is not the dominant process in the red giants either, at least not in 47 Tuc.  The CNO‐processed material seems most likely to have come from intermediate‐mass stars on the asymptotic giant branch; various mechanisms by which such material might appear in the main‐sequence stars are discussed. Explanations involving a truly primordial origin, or having successive generations of stars, seem to require rather special ‘tuning’. A somewhat speculative alternative self‐enrichment process is proposed, in which stellar winds from intermediate‐mass stars were captured by existing low‐mass stars, early in the life of the cluster. Whatever the mechanism, the abundance data presumably contain important clues about the formation and early evolution of globular clusters.

Using WFPC2 on the Hubble Space T elescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed, and sizable WD populations were found in all three. The location of these WDs in the colormagnitude diagram, their mean mass of 0.51(^0.03) and their luminosity function conÐrm basic M _ , tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and is found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are D9 Gyr old, a level that is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turno † ages of globular clusters and hence constrain the age of the universe.

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A new grid of theoretical color indices for the Strömgren uvby photometric system has been derived from MARCS model atmospheres and SSG synthetic spectra for cool dwarf and giant stars having À3:0 ½Fe=H þ0:5 and 3000 T eA 8000 K. At warmer temperatures (i.e., 8000 < T eA 40; 000 K), this grid has been supplemented with the synthetic uvby colors from recent Kurucz atmospheric models without overshooting (Castelli, Gratton, & Kurucz, published in 1997). Our transformations appear to reproduce the observed colors of extremely metal-poor turnoff and giant stars: the various uvby color-magnitude diagrams (CMDs) for the ½Fe=H $ À2:2 globular cluster M92 can be matched exceedingly well down to M V % 6 by the same isochrone that provides a very good fit to published BV data (see Paper I), on the assumption of the same distance and reddening. Due to a number of assumptions made in the synthetic color calculations, however, our color-T eff relations for cool stars fail to provide a suitable match to the uvby photometry of both cluster and field stars having ½Fe=H > À2:0. To overcome this problem, the theoretical indices at intermediate and high metallicities have been corrected using a set of color calibrations based on field stars having well-determined distances from Hipparcos, accurate T eff estimates from the infrared flux method, and spectroscopic [Fe/H] values. In contrast with Paper I, star clusters played only a minor role in this analysis in that they provided a supplementary constraint on the color corrections for cool dwarf stars with T eA 5500 K. They were mainly used to test the color-T eff relations and, encouragingly, isochrones that employ the transformations derived in this study are able to reproduce the observed CMDs (involving uÀv, vÀb, and bÀy colors) for a number of open and globular clusters (including M67, the Hyades, and 47 Tuc) rather well. Moreover, our interpretations of such data are very similar, if not identical, with those given in Paper I from a consideration of BV(RI ) C observations for the same clusters-which provides a compelling argument in support of the color-T eff relations that are reported in both studies. In the present investigation, we have also analyzed the observed Strömgren photometry for the classic Population II subdwarfs, compared our ''final'' (bÀy)-T eff relationship with those derived empirically in a number of recent studies and examined in some detail the dependence of the m 1 index on [Fe/ H].

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