Libraries of stellar spectra are fundamental tools for the study of stellar populations, and both empirical and synthetic libraries have been used for this purpose. In this paper, a new library of high resolution synthetic spectra is presented, ranging from the near-ultraviolet (300 nm) to the near-infrared (1.8 µm). The library spans all the stellar types that are relevant to the integrated light of old and intermediate-age stellar populations in the involved spectral region (spectral types F through M and all luminosity classes). The grid was computed for metallicities ranging from [Fe/H] = -2.5 to +0.5, including both solar and α-enhanced ([α/Fe] = 0.4) chemical compositions. The synthetic spectra are a good match to observations of stars throughout the stellar parameter space encompassed by the library and over the whole spectral region covered by the computations.

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Photometric data in the UBV(RI) C system have been acquired for 80 solar analog stars for which we have previously derived highly precise atmospheric parameters T eff , log g, and [Fe/H] using high-resolution, high signal-to-noise ratio spectra. UBV and (RI) C data for 46 and 76 of these stars, respectively, are published for the first time. Combining our data with those from the literature, colors in the UBV(RI) C system, with 0.01 mag precision, are now available for 112 solar analogs. Multiple linear regression is used to derive the solar colors from these photometric data and the spectroscopically derived T eff , log g, and [Fe/H] values. To minimize the impact of systematic errors in the model-dependent atmospheric parameters, we use only the data for the 10 stars that most closely resemble our Sun, i.e., the solar twins, and derive the following solar colors: (B − V ) = 0.653 ± 0.005, (U − B) = 0.166 ± 0.022, (V − R) = 0.352 ± 0.007, and (V − I ) = 0.702 ± 0.010. These colors are consistent, within the 1σ errors, with those derived using the entire sample of 112 solar analogs. We also derive the solar colors using the relation between spectral-line-depth ratios and observed stellar colors, i.e., with a completely model-independent approach, and without restricting the analysis to solar twins. We find (B − V ) = 0.653 ± 0.003, (U − B) = 0.158 ± 0.009, (V − R) = 0.356 ± 0.003, and (V − I ) = 0.701 ± 0.003, in excellent agreement with the model-dependent analysis.

We develop a new set of models for intermediate-metallicity single stellar populations in the blue/optical region and use those models to determine the spectroscopic age of 47 Tuc. The models are based on a moderately high-resolution (1.8Å FWHM) empirical spectral library, state-of-the-art theoretical isochrones from M. Salaris and the most recent set from the Padova group, and new semiempirical calibrations between fundamental stellar parameters and observables. Model line-strengths include all corrections for deficiencies of the stellar library that are described in Paper I. We highlight the importance of correctly modeling the luminosity function (LF) of the cluster at the level of the giant branch, in order to achieve a good reproduction of the integrated spectrum; agreement between the spectroscopic age and the age based on the cluster's color-magnitude diagram (CMD) is achieved only if the observed LF is used rather than the theoretical ones, which either do not include AGB stars (Salaris) or underpredict the

We perform an empirical synthesis of the blue integrated spectrum of the metal-rich globular cluster 47 Tucanae, based directly on the color-magnitude diagram of the cluster coupled to a moderately high-resolution spectral library. Freed from any significant dependence on theory, we are able to perform a fundamental test of the adequacy of the spectral library and its associated stellar parameters. Excellent fits are achieved for almost all absorption-line indices studied, provided the computations are corrected for two limitations of the spectral library, namely, the lack of a representative set of metal-poor giants and the absence of CN-strong stars. The latter effect is corrected by means of spectrum synthesis from model photospheres, considering the abundance pattern of CNstrong and CN-normal stars. We also need to perform a slight correction of the metallicity of the cluster (by -0.05 dex in relation to the standard value [Fe/H]=-0.7) in order to match the metal-line index measurements in the cluster spectrum.After these relatively small adjustments, the overall spectral agreement is good. Good fits are achieved for Hβ, Hγ, Mg b, < F e >, Ca4227 and Fe4383, and only Hδ F is overpredicted. Thus, ages inferred from Hδ F are slightly older than the ages based on the other Balmer lines, by ∼ 3 Gyrs. The success of this exercise suggests that previous failures to synthesize the spectrum of 47 Tuc must have arisen from inadequacies in the theoretical evolutionary isochrones and/or luminosity functions. Such a possibility is considered in a companion paper.