A STANDARD STELLAR LIBRARY FOR EVOLUTIONARY SYNTHESIS .1. CALIBRATIONOF THEORETICAL SPECTRA

We present a comprehensive hybrid library of synthetic stellar spectra based on three original grids of model atmosphere spectra by Kurucz ( 1995), Fluks et al. (1994), and Bessell et al. (1989, 1991), respectiv ely. The combined library has been intended for multiple-purpose synth etic photometry applications and was constructed according to the prec epts adopted by Buser & Kurucz (1992): (i) to cover the largest possib le ranges in stellar parameters (T-eff, log g, and [M/H]); (ii) to pro vide flux spectra with useful resolution on the uniform grid of wavele ngths adopted by Kurucz (1995); and (iii) to provide synthetic broad-b and colors which are highly realistic for the largest possible paramet er and wavelength ranges. Because the most astrophysically relevant st ep consists in establishing a realistic library, the corresponding col or calibration is described in some detail. Basically, for each value of the effective temperature and for each wavelength, me calculate the correction function. that must be applied to a (theoretical) solar-ab undance model flux spectrum in order for this to yield synthetic U BV RIJHKL 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 observ ations can indeed be eliminated. On the other hand, synthetic UBV and Washington ultraviolet excesses delta((U-B)) and delta((C-M)) and delt a((C-T1)) obtained from the original giant and dwarf model spectra are in excellent accord with empirical metal-abundance calibrations (Leje une & Buser 1996). Therefore, the calibration algorithm is designed in such a way as to preserve the original differential grid properties i mplied 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 temperatur e but different chemical compositions [M/H] and/or surface gravities l og g. While the new library constitutes a first-order approximation to the program set out above, it will be allowed to develop toward the m ore ambitious goal of matching the full requirements imposed on a stan dard library. Major input for refinement and completion is expected fr om the extensive tests now being made in population and evolutionary s ynthesis studies of the integrated light of globular clusters (Lejeune 1997) and galaxies (Bruzual et al. 1997).