Standard stellar library for evolutionary synthesis. I. Calibration of theoretical spectra

We present a comprehensive hybrid library of synthetic stellar spectra based on three original grids of model atmosphere spectra by [Kurucz (1995)]{ref26}, [Fluks et al. (1994)]{ref19}, and [Bessell et al. (1989, 1991)]{ref6}, respectively. The combined library has been intended for multiple-purpose synthetic photometry applications and was constructed according to the precepts adopted by [Buser & Kurucz (1992)]{ref14}: (i) to cover the largest possible ranges in stellar parameters (T_eff, log g, and [M/H]); (ii) to provide flux spectra with useful resolution on the uniform grid of wavelengths adopted by [Kurucz (1995)]{ref26}; and (iii) to provide synthetic broad--band colors which are highly realistic for the largest possible parameter and wavelength ranges. Because the most astrophysically relevant step consists in establishing a realistic library, the corresponding color calibration is described in some detail. Basically, for each value of the effective temperature and for each wavelength, we calculate the correction function that must be applied to a (theoretical) solar-abundance model flux spectrum in order for this to yield synthetic UBVRIJHKL colors matching the (empirical) color-temperature calibrations derived from observations. In this way, the most important systematic differences existing between the original model spectra and the observations can indeed be eliminated. On the other hand, synthetic UBV and Washington ultraviolet excesses delta_{(U-B)} and delta_{(C-M)} and delta_{(C-T₁)} obtained from the original giant and dwarf model spectra are in excellent accord with empirical metal-abundance calibrations ([Lejeune & Buser 1996]{ref30}). Therefore, the calibration algorithm is designed in such a way as to preserve the original differential grid properties implied by metallicity and/or luminosity changes in the new library, if the above correction function for a solar-abundance model of a given effective temperature is also applied to models of the same temperature but different chemical compositions [M/H] and/or surface gravities log g. While the new library constitutes a first-order approximation to the program set out above, it will be allowed to develop toward the more ambitious goal of matching the full requirements imposed on a standard library. Major input for refinement and completion is expected from the extensive tests now being made in population and evolutionary synthesis studies of the integrated light of globular clusters ([Lejeune 1997]{ref29}) and galaxies ([Bruzual et al. 1997]{ref10}).