2. The basic stellar libraries

The different libraries used are from Kurucz (1995), Bessell et al. (1989, 1991), and Fluks et al. (1994) - which we shall henceforth call the K-, B-, and F-libraries, respectively. Although the K-library covers a very wide temperature range (from to 3500 K), it does not extend to the very low temperatures required to model cool AGB stars. These stars are very important for population and evolutionary synthesis, since they can represent up to 40% of the bolometric and even up to 60% of the K-band luminosity of a single stellar generation (Bruzual & Charlot 1993). It is natural, then, to provide the necessary supplement by employing the suitable libraries that were specifically calculated for M-giant stars in the temperature range by Bessell et al. (1989, 1991) and by Fluks et al. (1994).

Table 1 (click here) summarizes the coverage of parameters and wavelengths provided by these three libraries, and Figs. 1 (click here) and 2 (click here) illustrate original sample spectra as functions of metallicity for two temperatures.

**Table 1:** Parameter and wavelength coverage of the different libraries
	Kurucz (1995)	Bessell et al. (1989, 1991)	Fluks et al. (1994)

			red giant sequence
[M/H]			solar
(nm)
	1221	705	10912

Figure 1: K-library dwarf model spectra for a range in metallicity. All the spectra are normalized at

Figure 2: B-library giant model spectra for a range in metallicity. All the spectra are normalized at

We should note the following: first, while both the K- and the B-library spectra are given for different but overlapping ranges in metallicity, the F-library spectra have been available for solar abundances only; secondly, the B-spectra are given for wavelengths and nonuniform sampling, while the F-spectra also cover the ultraviolet wavelengths in a uniform manner.

As it is one of the goals of the present effort to eventually also allow the synthesis of the metallicity-sensitive near-ultraviolet colors (e.g., the Johnson U and B), the B-library spectra - which account for the spectral changes due to variations in metallicity - have been complemented by the F-library spectra at ultraviolet wavelengths.

This step was achieved with the following procedure:

-: All the F- and B-spectra have first been resampled at the (same) uniform grid of wavelengths given by the K-library spectra.
-: F-spectra were then recomputed for the effective temperatures associated with the B-spectra. This was done by interpolation of the F-library sequence of 11 spectra representing M-giants of types M0 through M10, and using the spectral type - scale defined by Fluks et al.
-: Finally, in order to avoid the spurious spikes at present in the synthetic B-spectra (Worthey 1994), each B-spectrum was combined with the blue part () of the corresponding F-spectrum, i.e., having the same and having been rescaled to the B-spectrum flux level at .

The hybrid spectra created in this manner will hereafter be called "-spectra".

Of course in this way, completeness in wavelength coverage can only be established for solar-abundance models. However, extensions of the B-spectra to optical wavelengths down to the atmospheric limit at are being worked out now and will supersede the corresponding preliminary hybrid -spectra (Buser et al. 1997).

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